Optical information recording medium having super-resolution film

ABSTRACT

An optical information recording medium has a first information recording layer ( 20 ) and a second information recording layer ( 40 ) each of which includes (i) a group of pre-pits ( 31, 51 ) constituting marks ( 32, 52 ) and spaces ( 33, 53 ) and (ii) a super-resolution film ( 23, 43 ), the marks ( 32, 52 ) and the spaces ( 33, 53 ) having different lengths, an average length of a smallest mark that is smallest in length and a smallest space that is smallest in length being less than or equal to a resolution limit of a reproduction optical system for reproducing information recorded on the first information recording layer ( 20 ) and the second information recording layer ( 40 ), the group of pre-pits ( 31, 51 ) being formed so that a push-pull signal for the reproduction optical system to reproduce the information recorded by the group of pre-pits is negative in polarity. This provides an inexpensive and high-capacity multilayer optical information recording medium based on a super-resolution technology.

TECHNICAL FIELD

The present invention relates to an optical information recording mediumfor optically recording and reproducing information.

BACKGROUND ART

In recent years, optical information recording media have been requiredto have larger and larger information recording capacities forprocessing of a vast amount of information such as images. Solutions tothis are a method that involves the use of a super-resolutiontechnology, which is a technology for improving information processingduring reproduction, and a method that involves the use of a multilayeroptical information recording medium having multiple informationrecording layers on each of which recording/reproduction can beperformed.

The super-resolution technology is a technology for reproducing a signalof a mark length (determined according to a laser wavelength and thenumerical aperture of an optical system) that is less than or equal toan optical resolution limit of a reproducing device. Since this makes itpossible to perform recording with a smaller mark length, there is anincrease in substantive recording density. This is attributed to thefact that it is not a recording technology but a reproduction technologythat matters in terms of achieving a higher density.

Of these technologies, the super-resolution technology is describedfirst.

Conventionally, there have been proposed a large number of opticalinformation recording media (hereinafter referred to as“super-resolution media”) for reproducing a signal of a mark length thatis shorter than an optical system resolution limit of a reproducingdevice.

A known example of such a technology cannot be utilized for thereproduction of non-rewritable information recorded on a read-onlymedium with depressions in and protrusions on a substrate, but iscompatible with a rewritable optical magnetic recording medium having arecording film made of a magnetic material and is used in reproducinginformation recorded on the recording film along a magnetizationdirection (see Patent Literature 1).

Another known example of such a technology is compatible with aread-only medium as well as a rewritable optical recording medium, andprovides, on a reproduction light incidence surface of a reflectingfilm, a mask layer constituted by a thermochromic pigment layer thatchanges in optical characteristic (transmittance) according totemperature (see Patent Literature 2).

As will be described later, the mask layer is a layer that causes asuper-resolution phenomenon, for example, by pseudo-narrowing a laserspot.

Each of these optical information recording media utilizes a temperaturedistribution attributed to a light intensity distribution in a laserspot formed by a reproduction laser striking a reproduction surface ofthe optical information recording medium.

More specifically, such an optical magnetic recording medium as thatdisclosed in Patent Literature 1 has a reproduction layer provided on arecording layer. Moreover, during reproduction, a magnetic field of therecording layer is transferred onto the reproduction layer only in ahigh-temperature portion within a laser spot. This makes it possible asa result to reproduce a signal of a mark length that is shorter than theoptical resolution limit.

Further, in such an optical recording medium as that disclosed in PatentLiterature 2, there occurs a temperature or light intensity distributionwithin a reproduction laser spot on the reproduction layer that is closeto the reproduction light incident surface than the reflecting layer,whereby there occurs a distribution of optical characteristics withinthe laser spot.

For example, in a case where the reproduction layer is made of amaterial that becomes higher in transmittance as temperature rises, thereproduction layer becomes higher in transmittance in a high-temperatureportion, so that a laser spot occurring on the reflecting layer ispseudo-narrowed. This makes it possible as a result to reproduce asignal of a mark length that is shorter than the optical resolutionlimit.

However, since the super-resolution reproduction technologypseudo-narrows a laser spot, there is a decrease in efficiency in theuse of reproduction light (there is of course a decrease in reproductionlight). This imposes a limit on the narrowing of a laser spot, and animprovement in recording density is at most approximately twice as highin terms of liner density.

Next, a multilayer optical information recording medium is described.

As disclosed in Patent Literature 3, for example, a multilayer opticalinformation recording medium is structured to have (i) informationrecording layers such as a first information recording layer and asecond information recording layer provided in this order from areproduction light incident surface and (ii) an intermediate layer, mademainly of resin, which separates the information layers from each other.

In such a structure, an information recording layer other than theinformation recording layer located furthest from the reproduction lightincident surface is a half-transparent layer that transmits reproductionlight. This allows reproduction light incident on the reproduction lightincident surface to be focused on each information recording layer.Therefore, this multilayer optical information recording medium can besaid to be an optical information recording medium whose informationrecording density can increase as the total number of informationrecording layers increases.

It should be noted that the most common optical information recordingmedium that is used for this technology is a single-sided reproductiontwo-layer DVD-ROM.

However, an increase in the number of layers of a multilayer opticalinformation recording medium makes it difficult to produce themultilayer optical information recording medium, thus making the mediumvery expensive. The following explains a reason for the difficulty inproduction with reference to an example of a method for producing amultilayer optical recording medium.

In the production of a multilayer optical information recording medium,e.g., a DVD, which is currently most common, it is only necessary toform a first information recording layer such as a recording film and areflecting film on a substrate in vacuum, place back the resultingsubstrate into the atmosphere, and then join on top of the substrate asubstrate on which a second information recording layer has been formedin a similar manner, as long as the number of layers is up to two.

However, in a case where the number of layers increases to three, e.g.,in a case where the number of layers is three, the first informationrecording layer formed in vacuum is spin-coated in the atmosphere withan ultraviolet curing resin or the like that is to serve as anintermediate layer. Next, by joining a plastic stamper on top of theultraviolet curing resin in the atmosphere, curing the ultravioletcuring resin by irradiating it with ultraviolet rays, and then removingthe stamper, (i) grooves for tracking and (ii) depressions andprotrusions such as pre-pits on the basis of whose sequence informationhas been recorded are transferred onto a surface of the intermediatelayer (called “2P method”).

Then, again in vacuum, a second information recording film needs to beformed on the intermediate layer onto which the depressions andprotrusions such as the pre-pits have been transferred; furthermore, inthe atmosphere, a substrate on which a third information recording layerhas been formed needs to be prepared in a similar manner to thesubstrate provided with the first information recording layer, and bejoined on top of the second information recording layer.

In this way, the medium is produced through very complicated steps thatrequire the medium to move between vacuum and the atmosphere.

Further, since each layer has a different film structure for adjustingits reflectance and, during normal mass production, is formed on eachmedium traveling in one direction through the production line, as manyvacuum film-forming apparatuses as information recording layers arerequired. Moreover, such film-forming apparatuses are very expensive,and are high in running cost among apparatuses that are used in theproduction of optical information recoding media. Therefore, it has beensubstantially difficult in terms of cost to increase the number ofinformation recording layers to three or larger.

Despite the above two methods thus proposed as main methods forincreasing the density of an optical information recording medium, therehave been problems with each of these methods as mentioned above. Forthis reason, a multilayer super-resolution technology has come to beproposed as a technology which has the advantages of both methods andwhich can effectively increase the information recording density of anoptical information recording medium (Patent Literature 4).

CITATION LIST Patent Literature 1

-   Japanese Patent Application Publication, Tokukaihei, No. 8-180486 A    (Publication Date: Jul. 12, 1996)

Patent Literature 2

-   Japanese Patent Application Publication, Tokukai, No. 2001-035012 A    (Publication Date: Feb. 9, 2001)

Patent Literature 3

-   Japanese Patent Application Publication, Tokukai, No. 2000-235733 A    (Publication Date: Aug. 29, 2000)

Patent Literature 4

-   Japanese Patent Application Publication, Tokukai, No. 2006-269040 A    (Publication Date: Oct. 5, 2006)

SUMMARY OF INVENTION Technical Problem

However, as a result of a study, the inventors of the presentapplication found that a problem which does not occur in a multilayersuper-resolution read-only optical information recording medium for usein multilayer super-resolution reproduction that is a multilayer(non-super-resolution) read-only optical information recording medium ora multilayer super-resolution read-only optical information recordingmedium based on a monotone pattern recording method (recording based onmarks of the same length) occurs in a multilayer super-resolutionread-only optical information recording medium based on a mark edgerecording method which is often used in optical information recordingmedia as means for achieving a higher density in terms of signalprocessing.

The above problem is discussed below.

FIG. 19 is a schematic view showing a configuration of a read-onlytwo-layer DVD-ROM.

As mentioned above, as shown in FIG. 19, a single-sided read-onlytwo-layer DVD-ROM 902, which is most common as a multilayer opticalinformation recording medium, has a second substrate 960, a firstinformation recording layer 920, an intermediate layer 930, a secondinformation recording layer 940, and a substrate 950 stacked in thisorder from a side close to a reproduction light incident surface, and(i) the recording format of pre-pits (group of pre-pits 931) provided ata surface of contact (interface) between a side of the first informationrecording medium 920 that is closer to the reproduction light incidentsurface and the intermediate layer 930 was different from (ii) therecording format of (group of pre-pits 951) provided at a surface ofcontact between a side of the second information recording medium 940that is further from the reproduction light incident surface and thesubstrate 950.

Specifically, from the point of view of making a master, the recordingformat of the pre-pits in the first information recording layer 920 issuch that information is recorded on the first information recordinglayer 920 in an on-pit format (that forms the pre-pits so that thepre-pits are raised with respect to the reproduction light incidentsurface), and the recording format of the pre-pits in the secondinformation recording layer 940 is such that information is recorded onthe second information recording layer 940 in an in-pit format (thatforms the pre-pits so that the pre-pits are depressed with respect tothe reproduction light incident surface).

The above problem is a problem that occurs in a case where thisstructure is used in a multilayer super-resolution optical informationrecording medium based on the mark edge recording method. This problemwill be discussed in detail later. However, in such a structure, theproblem takes the form of a remarkable deterioration in reproducingcharacteristic of one of the information recording layers.

Such a problem does not normally occur in a group of pre-pits (markposition recording method) in a monotone pattern for which the mark edgerecording method is not used, as in the case of the groups of pre-pits931 and 951 shown in (a) and (b) of FIG. 20.

The present invention has been made in view of the foregoing problems,and it is an object of the present invention to provide an inexpensiveand high-capacity multilayer optical information recording medium basedon a super-resolution technology.

Solution to Problem

In order to solve the foregoing problems, an optical informationrecording medium of the present invention includes: a lighttransmittable layer having an incident surface on which reproductionlight is incident; two or more information recording layers; asubstrate, the light transmittable layer, the information recordinglayers, and the substrate being stacked in this order from an incidentside on which the reproduction light is incident; and an intermediatelayer that separates the information recording layers from each other,the two or more information recording layers having information recordedthereon as marks and spaces by a predetermined modulation method, thetwo or more information recording layers each including (i) a group ofpre-pits constituting the marks and the spaces and (ii) asuper-resolution film, the marks and the spaces constituted by the groupof pre-pits having different lengths, an average length of a smallestmark that is smallest in length among the marks constituted by the groupof pre-pits and a smallest space that is smallest in length among thespace constituted by the group of pre-pits being less than or equal to aresolution limit of a reproduction optical system for reproducing theinformation recorded on the information recording layers, the resolutionfilm being a film that enables the reproduction optical system toreproduce information recorded by the group of pre-pits, the group ofpre-pits being formed so that a push-pull signal for the reproductionoptical system to reproduce the information recorded by the group ofpre-pits is negative in polarity.

The group of pre-pits here is constituted by a plurality of pre-pits.The pre-pits mean depressed and protruding shapes provided in and on thesubstrate and the intermediate layer.

In order to solve the foregoing problems, an optical informationrecording medium of the present invention includes: a lighttransmittable layer having an incident surface on which reproductionlight is incident; two or more information recording layers; asubstrate, the light transmittable layer, the information recordinglayers, and the substrate being stacked in this order from an incidentside on which the reproduction light is incident; and an intermediatelayer that separates the information recording layers from each other,the two or more information recording layers having information recordedthereon as marks and spaces by a predetermined modulation method, thetwo or more information recording layers each including (i) a group ofpre-pits constituting the marks and the spaces and (ii) asuper-resolution film, the marks and the spaces constituted by the groupof pre-pits having different lengths, an average length of a smallestmark that is smallest in length among the marks constituted by the groupof pre-pits and a smallest space that is smallest in length among thespace constituted by the group of pre-pits being less than or equal to aresolution limit of a reproduction optical system for reproducing theinformation recorded on the information recording layers, the resolutionfilm being a film that enables the reproduction optical system toreproduce information recorded by the group of pre-pits, the group ofpre-pits being in an in-pit format by which the marks are formed moredepressed than the spaces with respect to the incident surface on whichthe reproduction light is incident.

According to the foregoing configuration, the group of pre-pits is in anin-pit format by which the marks are formed more depressed than thespaces with respect to the incident surface on which the reproductionlight is incident. This makes it possible to, even if the marks and thespaces are disposed so that the average length of the smallest mark andthe smallest space is less than or equal to the resolution limit of thereproduction optical system, prevent deterioration of the reproducingcharacteristics of information that is obtained by the reproductionoptical system reproducing the information recorded on the informationrecording layers.

Advantageous Effects of Invention

An optical information recording medium of the present inventionincludes: a light transmittable layer having an incident surface onwhich reproduction light is incident; two or more information recordinglayers; a substrate, the light transmittable layer, the informationrecording layers, and the substrate being stacked in this order from anincident side on which the reproduction light is incident; and anintermediate layer that separates the information recording layers fromeach other, the two or more information recording layers havinginformation recorded thereon as marks and spaces by a predeterminedmodulation method, the two or more information recording layers eachincluding (i) a group of pre-pits constituting the marks and the spacesand (ii) a super-resolution film, the marks and the spaces constitutedby the group of pre-pits having different lengths, an average length ofa smallest mark that is smallest in length among the marks constitutedby the group of pre-pits and a smallest space that is smallest in lengthamong the space constituted by the group of pre-pits being less than orequal to a resolution limit of a reproduction optical system forreproducing the information recorded on the information recordinglayers, the resolution film being a film that enables the reproductionoptical system to reproduce information recorded by the group ofpre-pits, the group of pre-pits being formed so that a push-pull signalfor the reproduction optical system to reproduce the informationrecorded by the group of pre-pits is negative in polarity.

An optical information recording medium of the present inventionincludes: a light transmittable layer having an incident surface onwhich reproduction light is incident; two or more information recordinglayers; a substrate, the light transmittable layer, the informationrecording layers, and the substrate being stacked in this order from anincident side on which the reproduction light is incident; and anintermediate layer that separates the information recording layers fromeach other, the two or more information recording layers havinginformation recorded thereon as marks and spaces by a predeterminedmodulation method, the two or more information recording layers eachincluding (i) a group of pre-pits constituting the marks and the spacesand (ii) a super-resolution film, the marks and the spaces constitutedby the group of pre-pits having different lengths, an average length ofa smallest mark that is smallest in length among the marks constitutedby the group of pre-pits and a smallest space that is smallest in lengthamong the space constituted by the group of pre-pits being less than orequal to a resolution limit of a reproduction optical system forreproducing the information recorded on the information recordinglayers, the resolution film being a film that enables the reproductionoptical system to reproduce information recorded by the group ofpre-pits, the group of pre-pits being in an in-pit format by which themarks are formed more depressed than the spaces with respect to theincident surface on which the reproduction light is incident.

This brings about an effect of making it possible to provide aninexpensive and high-capacity optical information recording medium thatis prevented from deteriorating in reproducing characteristic.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view showing a configuration of an opticalinformation recording medium of the present invention.

FIG. 2 is an enlarged perspective view showing a configuration of agroup of pre-pits provided in an intermediate layer of the opticalinformation recording medium of the present invention.

FIG. 3 is an enlarged perspective view showing a configuration of agroup of pre-pits provided in a substrate of the optical informationrecording medium of the present invention.

FIG. 4 shows the appearance of a reproduction optical system push-pullsignal for a reproduction machine to reproduce information recorded bythe groups of pre-pits, the push-pull signal being negative in polarity.

FIG. 5 shows the appearance of a reproduction optical system push-pullsignal for the reproduction machine to reproduce information recorded bythe groups of pre-pits, the push-pull signal being positive in polarity.

FIG. 6 is a plan view showing a configuration of the optical informationrecording medium of the present invention.

FIG. 7 is a schematic view showing an example of a BCA disposed in theoptical information recording medium of the present invention.

FIG. 8 is a schematic view showing a configuration of an opticalinformation recording medium of the present invention.

FIG. 9 is a schematic view showing a configuration of an opticalinformation recording medium of a comparative example.

FIG. 10 shows results of evaluation of information recorded on a firstinformation recording layer of the optical information recording mediumof FIG. 8 and information recorded on an information recording layer ofthe optical information recording medium of FIG. 9.

FIG. 11 is a set of schematic views (a) and (b), (a) showing aconfiguration of an optical information recording medium 300A, (b)showing a configuration of an optical information recording medium 300B.

FIG. 12 is a set of schematic views (a) and (b), (a) showingconfigurations of optical information recording media 400A, 500A, and600A, (b) showing configurations of optical information recording media400B, 500B, and 600B.

FIG. 13 is a set of enlarged perspective views (a) and (b), (a) showinga configuration of a group of pre-pits in an in-pit format, (b) showinga configuration of a group of pre-pits in an on-pit format.

FIG. 14 shows results of measurement of reproduction signals ofexperimental optical information recording media.

FIG. 15 is a schematic view showing a configuration of an opticalinformation recording medium having pre-pits formed in a monotoneformat.

FIG. 16 is a set of enlarged perspective views (a) and (b), (a) showinga configuration of a group of pre-pits in a monotone on-pit format, (b)showing a configuration of a group of pre-pits in a monotone in-pitformat.

FIG. 17 shows the reproducing characteristics (OTF) of first and secondinformation recording layers of the optical information recording mediumof FIG. 15.

FIG. 18 shows a configuration of an optical information recording mediumwhose second information recording layer has a super-resolution filmconstituted by a single layer.

FIG. 19 is a schematic view showing a configuration of a conventionalread-only two-layer DVD-ROM.

FIG. 20 is a set of enlarged perspective views (a) and (b), (a) showinga configuration of a group of pre-pits in a monotone on-pit format, (b)showing a configuration of a group of pre-pits in a monotone in-pitformat.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention are described in detail below.

(1. Schematic Configuration of an Optical Information Recording Medium1)

A configuration of optical information recording medium (multilayersuper-resolution optical information recording medium) 1 according to anembodiment of the present invention is described.

FIG. 1 is a schematic view showing a configuration of the opticalinformation recording medium 1.

As shown in FIG. 1, the optical information recording medium 1 includes:a cover layer (light transmittable layer) 10 having an incident surfaceon which reproduction light is incident; two or more informationrecording layers, i.e. a first information recording layer (informationrecording layer) 20 and a second information recording layer(information recording layer) 40; a substrate 50, the lighttransmittable layer 10, the first and second information recordinglayers 20 and 40, and the substrate 50 being stacked in this order froman incident side on which the reproduction light is incident; and anintermediate layer 30 that separates the first information recordinglayer 20 and the second information recording layer 40 from each other.

The optical information recording medium 1 has a multilayer structureconstituted by two information recording layers, namely the firstinformation recording layer 20 and the second information recordinglayer 40. The first information recording layer 20 and the secondinformation recording layer 40 are formed on the intermediate layer 30provided with a group of pre-pits 31 and the substrate 50 provided witha group of pre-pits 51, respectively, and are read-only (ROM; Read OnlyMemory) information recording layers from which information can only beread out by reproduction light.

The first information recording layer 20 and the second informationrecording layer 40 have information recorded thereon in the form ofshapes by marks and spaces by a predetermined modulation method, and themarks and the spaces are constituted by the groups of pre-pits 31 and51.

The following description assumes that of the first and secondinformation recording layers 20 and 40, the second information recordinglayer 40, on which the substrate 50 is stacked, (i.e., the informationrecording layer that is further from the reproduction light incidentsurface) is sometimes referred to as “L0 layer”.

(Components of the Optical Information Recording Medium 1)

The components of the optical information recording medium 1 isdescribed in turn with reference to FIG. 1.

The cover layer 10 is located closest to the reproduction light incidentside among the layers constituting the optical information recordingmedium 1. The reproduction light incident surface is a surface of thecover layer 10 that is opposite to that surface of the cover layer 10which is in contact with the first information recording layer 20.

The cover layer 10 is made, for example, of an ultraviolet curing resinhaving a thickness of 75 μm (with a refractive index of 1.50 at areproduction light wavelength of 405 nm). The cover layer 10 needs onlybe made of a material that exhibits a high transmittance at thereproduction light wavelength. That is, the cover layer 10 may beformed, for example, by a polycarbonate film and a transparent adhesive.

Further, the cover layer 10 may have a surface with an antifoulingproperty (property that does not cause deterioration of reproductionsignals or the like from the first and second information recordinglayers 20 and 40 even in the case of contamination with fingerprints) oran abrasion-resistant property that does not affect reproduction. Itshould be noted that the antifouling property and the abrasion-resistantproperty may be attained by providing the surface of the cover layer 10with a hard coat.

Furthermore, the cover layer 10 may vary in thickness according to anoptical system (reproduction optical system) of a reproducing device forthe optical information recording medium 1. Specifically, the coverlayer 10 may for example be a polycarbonate substrate having a thicknessof 0.6 mm.

The first information recording layer 20 is a read-only informationrecording layer (ROM layer) provided adjacent to the cover layer 10.

The first information recording layer 20 includes (i) the group ofpre-pits 31 provided in the intermediate layer 30 put underneath thefirst information recording layer 20 and (ii) a super-resolution film 23stacked on the group of pre-pits 31.

The first information recording layer 20 has information recordedthereon in the form of shapes by the group of pre-pits 31 provided inthe intermediate layer 30 put underneath the first information recordinglayer 20. The group of pre-pits 31 provided in the intermediate layer 30is constituted by a plurality of pre-pits having depressed andprotruding shapes.

Stacking of the super-resolution film 23 on the group of pre-pits 31formed in depressed and protruding shapes in and on the intermediatelayer 30 causes the depressions and protrusions of the group of pre-pits31 to form the super-resolution film 23 into depressed and protrudingshapes. In this way, the first information recording layer 20 hasinformation recorded thereon in the form of shapes by the group ofpre-pits 31.

As will be mentioned later, the group of pre-pits 31 formed in theintermediate layer 30 is formed to be less than or equal to an opticalresolution limit (hereinafter sometimes referred to simply as“resolution limit”) of the reproduction optical system.

The super-resolution film 23 is a film that enables the reproductionoptical system to reproduce information recorded by the group ofpre-pits 31. That is, the super-resolution film 23 is a super-resolutionfilm that enables the reproduction optical system to performreproduction (super-resolution reproduction) even if the average lengthof the smallest mark and the smallest space in the group of pre-pits 31is less than or equal to the resolution limit.

The first information recording layer 20 is a layer, obtained bystacking the super-resolution film 23 on the group of pre-pits 31, whichenables the reproduction optical system to reproduce informationrecorded by the group of pre-pits 31.

The super-resolution film 23 is for example a film which allows a spotdiameter of incident reproduction light to be pseudo-narrowed by atemperature distribution of the reproduction light and which allowsinformation recorded by the group of pre-pits 31 to be reproduced by aphenomenon occurring in a super-resolution reproduction technology, suchas a super ROM, whose principle per se has yet to be explained.

The super-resolution film 23 is constituted by two thin films stacked bysputtering or the like. Specifically, these two thin films are areproduction film 21 (made of zinc oxide with a thickness ofapproximately 55 nm) and a reflecting film 22 (made of Ti with athickness of approximately 6 nm), the reproduction film 21 and thereflecting film 22 being stacked in this order from the reproductionlight incident side.

The material for the super-resolution film 23, the thickness of thesuper-resolution film 23, and the number of films by which thesuper-resolution film 23 is constituted are not to be limited to thosedescribed above. The super-resolution film 23 needs only be asuper-resolution film which functions as a layer that enablessuper-resolution reproduction and which enables reproduction ofinformation recorded by the group of pre-pits 31 provided in theintermediate layer 30, which will described later.

Further, the term “reproduction film” means a film that enablessuper-resolution reproduction by being combined with a semi-transparentfilm or a reflecting film, and the term “semi-transparent film” means afilm that transmits reproduction light and plays a role of a reflectingfilm.

The intermediate layer 30 is made, for example, of an ultraviolet curingresin having a thickness of 25 μm (with a refractive index of 1.50 atthe reproduction light wavelength). The material for the intermediatelayer 30 is not to be limited to such a material, but needs only be amaterial that exhibits a high transmittance at the reproduction lightwavelength. Further, the thickness of the intermediate layer 30 is notlimited to such a thickness, either, but needs only be an appropriatethickness that allows separation of the information recording layers(here, the first and second information recording layers 20 and 40) fromeach other and does not cause a problem of interlayer crosstalk.

The term “interlayer crosstalk” here means noise generated from aninformation recording layer other than an information recording layerwhose information is being reproduced. Alternatively, the intermediatelayer 30 may have a multilayer structure.

Provided in that surface of the intermediate layer 30 which is incontact with the super-resolution film 23 is the group of pre-pits 31formed by a 2P method (photo polymerization method) in depressed andprotruding shapes formed in accordance with information recorded asshaped on the first information recording layer 20. The group ofpre-pits 31 is formed in an in-pit format by which pre-pits are formedin depressed shapes with respect to the reproduction light incidentsurface.

The term “2P method” here means a method by which depressions in andprotrusions on (pre-pits in) a master are transferred onto a flap plateby (i) filling a space between the flat plate and the master with anultraviolet curing resin, (ii) curing the ultraviolet curing resin byirradiating it with ultraviolet rays, and (iii) removing the mater.

A configuration of the group of pre-pits 31 will be described later.

The second information recording layer 40 is a read-only informationrecording layer (ROM layer) provided in a position furthest from thecover layer 10.

The second information recording layer 40 includes (i) the group ofpre-pits 51 provided in the substrate 50 put underneath the secondinformation recording layer 40 and (ii) a super-resolution film 43stacked on the group of pre-pits 51.

The second information recording layer 40 has information recordedthereon in the form of shapes by the group of pre-pits 51 provided inthe substrate 50 put underneath the second information recording layer40. The group of pre-pits 51 provided in the substrate 50 is constitutedby a plurality of pre-pits having depressed and protruding shapes.

Stacking of the super-resolution film 43 on the group of pre-pits 51formed in depressed and protruding shapes in and on the substrate 50causes the depressions and protrusions of the group of pre-pits 51 toform the super-resolution film 43 into depressed and protruding shapes.In this way, the second information recording layer 40 has informationrecorded thereon in the form of shapes by the group of pre-pits 51.

As will be mentioned later, the group of pre-pits 51 formed in thesubstrate 50 is formed to be less than or equal to the opticalresolution limit of the reproduction optical system. Thesuper-resolution film 43 is a film that enables the reproduction opticalsystem to reproduce information recorded by the group of pre-pits 51.That is, the super-resolution film 43 is a super-resolution film thatenables the reproduction optical system to perform reproduction even ifthe average length of the smallest mark and the smallest space in thegroup of pre-pits 51 is less than or equal to the resolution limit.

The second information recording layer 40 is a layer, obtained bystacking the super-resolution film 43 on the group of pre-pits 51, whichenables the reproduction optical system to reproduce informationrecorded by the group of pre-pits 51.

The super-resolution film 43 is for example a film which allows a spotdiameter of incident reproduction light to be pseudo-narrowed by atemperature distribution of the reproduction light and which allowsinformation recorded by the group of pre-pits 51 to be reproduced by aphenomenon occurring in a super-resolution reproduction technology, suchas a super ROM, whose principle per se has yet to be explained.

The super-resolution film 43 is constituted by two thin films stacked bysputtering or the like. Specifically, these two thin films are areproduction film 41 (made of zinc oxide with a thickness ofapproximately 65 nm) and a reflecting film 42 (made of Ta with athickness of 7 nm), the reproduction film 41 and the reflecting film 42being stacked in this order from the reproduction light incident side.

The material for the super-resolution film 43, the thickness of thesuper-resolution film 43, and the number of films by which thesuper-resolution film 43 is constituted are not to be limited to thosedescribed above. The super-resolution film 43 needs only be asuper-resolution film which functions as a layer that enablessuper-resolution reproduction and which enables reproduction ofinformation recorded by the group of pre-pits 51 provided in thesubstrate 50, which will described later.

The substrate 50 is a disk-shaped substrate having a group of pre-pitson which information has been recorded in a similar in-pit format to theintermediate layer 30. The substrate 50 is for example a polycarbonatesubstrate having a diameter of 120 mm and a thickness of 1.1 mm.

The material for the substrate 50 and the thickness of the substrate 50are not to be limited to those described above, as long as the substrate50 has the group of pre-pits 51 provided in a surface (light incidentsurface) thereof and has such a predetermined degree of strength as tobe usable as a substrate. Specifically, the substrate 50 may be made ofpolyolefin resin, metal, or the like. Furthermore, the substrate 50 mayhave a multilayer structure.

In this way, the optical information recording medium 1 includes aplurality of information recording layer, namely the first informationrecording layer 20 and the second information recording layer 40.

Again, the first information recording layer 20 includes (i) the groupof pre-pits 31, in which the average length of the smallest mark and thesmallest space is less than or equal to the resolution limit of thereproduction optical system and (ii) the super-resolution film 23stacked on the group of pre-pits 31.

Similarly, the second information recording layer 40 includes (i) thegroup of pre-pits 51, in which the average length of the smallest markand the smallest space is less than or equal to the resolution limit ofthe reproduction optical system and (ii) the super-resolution film 43stacked on the group of pre-pits 51. In this way, the opticalinformation recording medium 1 is configured as a multilayersuper-resolution optical information recording medium.

(Modification of the Optical Information Recording Medium 1)

Next, a modification of the optical information recording medium 1 isdescribed with reference to FIG. 18.

The optical information recording medium 1 needs only be a multilayersuper-resolution optical information recording medium including two ormore information recording layers each of which includes (i) a group ofpre-pits in which the average length of the smallest mark and thesmallest space is less than or equal to the resolution limit of areproduction optical system and (ii) a super-resolution film that is afilm that enable the reproduction optical system to reproduceinformation recorded by the group of pre-pits.

That is, in a case where the groups of pre-pits 31 and are each made ofa metal material having a high reflectance, the super-resolution films23 and 43, which are stacked on the groups of pre-pits 31 and 51respectively, may each be constituted not by two layers but by a singlelayer.

An example of a configuration of such a multilayer super-resolutionoptical information recording medium is described with reference to FIG.18.

FIG. 18 is a schematic view showing a configuration of an opticalinformation recording medium including a plurality of informationrecording layers a second information recording layer 40 a of which hasa super-resolution film 43 constituted by a single layer.

An optical information recording medium 1 a includes a cover layer 10, afirst information recoding layer 20, an intermediate layer 30, a secondinformation recording layer 40 a, and a substrate 50, in this order froman incident side on which reproduction light is incident.

The second information recording layer 40 a includes (i) a group ofpre-pits 51 a and (ii) a super-resolution film 43 stacked on the groupof pre-pits 51 a.

The super-resolution film 43 is made of zinc oxide having a thickness ofapproximately 65 nm, as with the reproduction film 41.

The substrate 50 a is made of a metal material having a highreflectance. Examples of such a metal material include Ta and the like.The substrate 50 a has the group of pre-pits 51 a formed in a surfacethereof on which the super-resolution film 43 is stacked. The group ofpre-pits 51 a formed in the substrate 50 a is made of the same metalmaterial, such as Ta, as the substrate 50 a. This allows the group ofpre-pits 51 a to also function as a reflecting film.

It should be noted that the lengths of marks and spaces constituted bythe group of pre-pits 51 are the same as those of the group of pre-pits51.

The optical information recording medium 1 a, which is a multilayersuper-resolution optical information recording medium, may be configuredin this way.

In the optical information recording medium 1 a, the second informationrecording layer 40 a includes the group of pre-pits 51 a and thesuper-resolution film 43, with the group of pre-pits 51 a made of ametal material having a high reflectance and the super-resolution film43 constituted by a single layer.

Since the group of pre-pits 51 a is made of a metal material having ahigh reflectance, the super-resolution film 41 does not need to beconstituted by a plurality of layers stacked on top of each other. Thismakes it possible to reduce manufacturing costs as compared the case ofa super-resolution film stacked on the group of pre-pits 51 a of theoptical information recording medium 1.

(Configuration of the Groups of Pre-Pits)

Next, configurations of the groups of pre-pits 31 and 51 provided in theintermediate layer 30 and the substrate 50 respectively are describedwith reference to FIGS. 2 through 6.

FIG. 2 is a perspective view showing a configuration of pre-pitsprovided in the intermediate layer 30. FIG. 3 is a perspective viewshowing a configuration of pre-pits provided in the substrate 50.

The groups of pre-pits 31 and 51 are arrangements of a plurality ofpre-pits constituted by depressions and protrusions formed in accordancewith information recorded as shaped on the first and second informationrecording layers 20 and 40. The group of pre-pits 31 includes marks 32and spaces 33, and the group of pre-pits 51 includes marks 52 and spaces53. Each of the marks 32 and 52 is a reflecting surface of a pre-pitwhich reflecting surface reflects reproduction light, and each of thespaces 33 and 53 is a reflecting surface between pre-pits whichreflecting surface reflects reproduction light. The groups of pre-pits31 and 51 record information on the first and second informationrecording layers 20 and 40, respectively, in accordance with apredetermined modulation method, e.g., a 1-7RLL modulation method.

The marks 32 and 52 constituted by the groups of pre-pits 31 and 51 havedifferent lengths. By thus causing the marks 32 and 52 to have differentlengths, an improvement in density at which information is recorded canbe achieved as compared with a so-called monotone pattern recordingmethod where marks have the same length.

In the marks 32 and 52 thus configured to have different lengths,information is recorded by a mark edge recording method, for example, inaccordance with the 1-7RLL modulation method. The mark edge recordingmethod is a method by which information (“1” (high) or “0” (low) is readby reading an edge portion of each of the marks 32 and 52 (boundaryportion of each of the marks 32 and 52 with its adjacent space 33 or53).

Use of the mark edge recording method makes it possible to improve thevolume of information to be recorded, as compared with a mark positionrecording method used as the monotone pattern recording method.

The groups of pre-pits 31 and 51 are formed such that the average lengthof the smallest mark that is smallest in length among the marks 32(52)constituted by the groups of pre-pits 31(51) and the smallest space thatis smallest in length among the spaces 33(53) constituted by the groupsof pre-pits 31(51) is less than or equal to the resolution limit of areproduction optical system for reproducing the information recorded onthe information recording layers (the first and second informationrecording layers 20 and 40).

This makes it possible to improve the density at which the marks 32 and52 and the spaces 33 and 53 are disposed, thus making it possible torecord a large volume of information on each of the first and secondinformation recording layers 20 and 40.

The term “the average length of the smallest mark and the smallestspace” here means a length that can be calculated from a predeterminedmodulation method and the density of information recorded on the firstinformation recording layer 20 or the second information recording layer40.

Structurally, for example, in the case of the 1-7RLL modulation method,the average length is an average length of the smallest mark, 2T marklength, and the smallest space, 2T space length.

Specifically, in a case where the reproduction optical system is basedon BD (Blu-ray Disc), an average length of the smallest mark, 2T marklength, and the smallest space, 2T space length, in the 1-7RLLmodulation method is 93 nm, for example.

In addition, as described above, each of the super-resolution films 23and 43 is configured as a super-resolution film that enables thereproduction optical system to perform reproduction even if the groupsof pre-pits 31 and 51 are formed to be less than or equal to theresolution limit of the reproduction optical system. This allows thereproduction optical system to reproduce information recorded by thegroups of pre-pits 31 and 51 (i.e. information recorded by the marks 32and 52 and the spaces 33 and 53).

In this way, the optical information recording medium 1 is configured asa multilayer super-resolution optical information recording medium.

Specifically, the optical information recording medium 1 is configuredsuch that two information recording layers, namely the first and secondinformation recording layers 20 and 40, are disposed, and that thesuper-films 23 and 43 are stacked on the groups of pre-pits 31 and 51,respectively, in each of which the average length of the smallest markand the smallest space is less than or equal to the resolution limit ofthe reproduction optical system.

By thus configuring the optical information recording medium 1 as amultilayer super-resolution optical information recording medium, it ismade possible to prevent the number of information recording layers tobe stacked from further increasing to three layers, four layers, andmore for the purpose of increasing a capacity for recording informationon the information recording layers. Thus, by configuring the opticalinformation recording medium 1 as a multilayer super-resolution opticalinformation recording medium, it is made possible to increase a capacityfor storing information and to prevent a manufacturing cost increaseassociated with an increase in the number of information recordinglayers to be stacked.

Further, the groups of pre-pits 31 and 51 are formed in an in-pit formatby which the marks 32 and 52 are formed more depressed than the spaces33 and 53 with respect to the incident surface on which the reproductionlight is incident. A push-pull signal for the reproduction opticalsystem to reproduce the information recorded by the groups of pre-pits31 and 51 in the in-pit format is negative in polarity.

In other words, the groups of pre-pits 31 and 51 are formed such that apush-pull signal for the reproduction optical system to reproduce theinformation recorded by the groups of pre-pits 31 and 51 is negative inpolarity.

FIG. 4 shows the appearance of a reproduction optical system push-pullsignal for a reproduction machine to reproduce information recorded bythe groups of pre-pits, the push-pull signal being negative in polarity.

FIG. 5 shows the appearance of a reproduction optical system push-pullsignal for the reproduction machine to reproduce information recorded bythe groups of pre-pits, the push-pull signal being positive in polarity.

The phrase “push-pull signal being negative in polarity” means that in areproduction machine where a push-pull signal is calculated bysubtracting a voltage obtained by one light-receiving section, forreceiving a reproduction signal, disposed closer to the center of theoptical information recording medium 1 from a voltage obtained byanother light-receiving section, for receiving a reproduction signal,disposed closer to the outer edge of the optical information recordingmedium 1, the push-pull signal takes on a level of 0, a negative level,a level of 0, a positive level, and a level of 0 in this order, as shownin FIG. 4, when a jump to an inner track occurs from a track in whichinformation recorded by pre-pits is reproduced.

At a time when the reproduction machine reproduces information recordedby the groups of pre-pits 31 and 51 formed in the in-pit format on theoptical information recording medium 1, the polarity of a reproductionoptical system push-pull signal is negative, as shown in FIG. 4.

At a time when the reproduction machine reproduces information recordedby groups of pre-pits formed in an on-pit format, a push-pull signalwhose polarity is positive is obtained to take on a level of 0, apositive level, a level of 0, a negative level, a level of 0 in thisorder, as shown in FIG. 5.

Even with the marks and the spaces disposed so that the average lengthof the smallest mark and the smallest space both of which areconstituted by the groups of pre-pits 31 and 51 is less than or equal tothe resolution limit of the reproduction optical system, it is possibleto prevent deterioration in reproducing characteristic of informationthat is obtained by the reproduction optical system reproducinginformation recorded on each of the first and second informationrecording layers 20 and 40.

With the optical information recording medium 1, it is thereforepossible to realize a high-capacity and inexpensive optical informationrecording medium that is prevented from deteriorating in reproducingcharacteristic.

That is, the configuration of the optical information recording medium 1makes it possible to impart satisfactory reproducing characteristics toall of the information recording layers formed in the opticalinformation recording medium 1, i.e., to the first information recordinglayer 20 and the second information recording layer 40.

Furthermore, it is preferable that the average length of the smallestmark and the smallest space in the group of pre-pits 31 (51) be lessthan or equal to λ/(5.76 NA), where λ is the reproduction lightwavelength of a reproduction optical system that reproduced informationrecorded on the optical information recording medium 1 and NA is thenumerical aperture of the reproduction optical system.

In a case where the average length of the smallest mark and the smallestspace in the group of pre-pits is greater than λ/(5.76 NA), thegeneration of clocks for reproducing information recorded on theinformation recording layer is possible even if the recording format ofthe group of pre-pits is an on-pit format, as will be mentioned later.

This makes it possible to obtain necessary reproducing characteristicsby complementing reproducing characteristics even if the reproducingcharacteristics deteriorate. That is, when the average length of thesmallest mark and the smallest space in the group of pre-pits is lessthan or equal to λ/(5.76 NA), there is a remarkable difference inreproducing characteristic depending on the recording format (in-pitrecording format or on-pit recording format) of the group of pre-pits.

In this case, there is a more remarkable difference depending on therecording format of pits, and a further increase in capacity can beachieved by forming pits in an in-pit format.

That is, in the case of an optical information recording medium producedin a configuration different from the optical information recordingmedium 1, it is extremely difficult to correct reproducingcharacteristics of an information recording layer when the averagelength of the smallest mark and the smallest space in a group ofpre-pits is less than λ/(5.76 NA).

On the contrary, the optical information recording medium 1 isconfigured such that each of the groups of pre-pits 31 and 51 is formedin an in-pit format. Therefore, even when the average length of thesmallest mark and the smallest space in each of the groups of pre-pits31 and 51 is less than λ/(5.76 NA), it is possible to eliminate the needfor correction of the reproducing characteristics of informationobtained by reproducing information recorded on the first and secondinformation recording layers 20 and 40, and to thus preventdeterioration in reproducing characteristic and obtain satisfactoryreproducing characteristics.

(Planar Configuration of the Optical Information Recording Medium 1)

Next, a planar configuration of the optical information recording medium1 will be described with reference to FIGS. 6 and 7.

FIG. 6 is a plan view schematically showing a configuration of theoptical information recording medium 1.

FIG. 7 is a schematic view showing an example of a BCA disposed in theoptical information recording medium 1. In FIG. 7, the arrow arepresents length, and the arrow b represents width.

As shown in FIG. 6, the optical information recording medium 1 includes:a region A (content recording region), which is a region in which toperform general information recording and reproduction; and a region B,which is a region in which to record and reproduce a BCA (burst cuttingarea).

The region A is a region in which to record reproduction contentinformation and the like for a user to use. The region A is aninformation recording region that requires tracking for reproducingrecorded information.

The region B is a BCA recording region for recording BCA, and isprovided closer to the center of the optical information recordingmedium 1.

As shown in FIG. 7, the BCA takes the form of a barcode.

The BCA is recorded in the second information recording layer 40 by arecording method that renders the BCA more easily detectable than theinformation recorded as the marks 32 and 52 and the spaces 33 and 53(i.e. the groups of pre-pits 31 and 51) on the first and secondinformation recording layers 20 and 40.

The BCA contains the following pieces of information: (i) disk-typeidentification information, which is information for identifying a disktype (medium type such as DVD or BD) of the optical informationrecording medium 1; and (ii) individual identification information,which is information for individually identifying the opticalinformation recording medium 1.

Furthermore, the disk-type identification information also containsinformation indicating that the first information recording layer 20 andthe second information recording layer 40 are layers configured to allowa reproduction optical system to reproduce information recorded by thegroups of pre-pits 31 and 51. That is, the disk-type identificationinformation also contains information indicating that the opticalinformation recording medium 1 is a multilayer super-resolution opticalinformation recording medium.

The BCA is formed in a L0 layer (second information recording layer 40),which is an information recording layer provided in a position furthestfrom the cover layer 10. That is, in the optical information recordingmedium 1, the disk-type identification information and the individualidentification information are recorded on the L0 layer, which is aninformation recording layer provided at a position furthest from thecover layer 10.

This makes it possible to preset, in a reproducing device thatreproduces the disk-type identification information and the individualidentification information, which of a plurality of informationrecording layers, i.e. the first information recording layer 20 and thesecond information recording layer 40 has the disk-type identificationinformation and the individual identification information recordedthereon.

The above configuration makes it possible to reduce the amount of timerequired for causing the reproducing device to identify the secondinformation recording layer 40 as a layer where the disk-typeidentification information and the individual identification informationare recorded, among a plurality of information recording layers, i.e.the first information recording layer 20 and the second informationrecording layer 40. Furthermore, the above configuration facilitates useof optical information recording media of various disk types by a singlereproducing device.

It should be noted that the BCA is formed in stripes having widths inunits of 10 μm in the second information recording layer 40, which isthe L0 layer, by irradiating the second information recording layer 40with pulse laser light during the manufacture of the optical informationrecording medium 1.

In this way, the BCA is formed on the second information recording layer40. Since the BCA takes the form of stripes having widths in units of 10μm and lengths in units of 100 μm to several hundreds micrometers (inunits of mm), the BCA is larger than the groups of pre-pits 31 and 51(the marks 32 and 52 and the spaces 33 and 53) respectively formed inthe first information recording layer 20 and the second informationrecording layer 40.

This allows a reproducing device which reproduces the BCA to read theBCA without tracking even if focusing of the reproducing device isslightly off. Thus, the BCA is formed on the second informationrecording layer 40 by a recording method that renders the BCA moreeasily detectable than a recording method for recording information asthe marks 32 and 52 and the spaces 33 and 53.

Further, use of a dedicated pulse laser light irradiation device makesit possible to comparatively easily form a BCA as disk-typeidentification information and an individual identification number inthe optical information recording medium 1.

Further, as described above, the marks 32 and 52 respectively formed onthe first and second information recording layers 20 and 40 of theoptical information recording medium 1, which is a multilayersuper-resolution optical information recording medium, are formed at asuch high density that the average length of the smallest mark and thesmallest space is less than or equal to the resolution limit of thereproduction optical system.

In the case of reproduction of information recorded at a high density,like the information recorded on the first and second informationrecording layers 20 and 40, it is necessary to make reproduction laserpower (intensity of the reproduction light) larger than in the case ofreproduction of information recorded on an optical information recordingmedium (e.g. non-multilayer super-resolution optical informationrecording medium in which the average length of the smallest mark andthe smallest space is greater than the resolution limit of areproduction optical system) having an information recording layerformed at a lower density than the first and second informationrecording layers 20 and 40.

For this reason, an attempt to reproduce information on thenon-multilayer super-resolution optical information recording medium byreproduction laser power by which to reproduce information on theoptical information recording medium 1 may result in destruction of thenon-multilayer super-resolution optical information recording medium.

However, with an arrangement in which the disk-type identificationinformation is recorded on the second information recording layer 40 bya recording method that renders information, like the BCA, more easilydetectable than a recording method for recording information as themarks 32 and 52 and the spaces 33 and 53, it is possible to determinewhether or not the optical information recording medium is a multilayersuper-resolution optical information recording medium by confirmingdisk-type identification information before increasing reproductionlaser power by which to reproduce information recorded as the marks 32and 52 and the spaces 33 and 53.

This prevents information on a non-multilayer super-resolution opticalinformation recording medium from being mistakenly reproduced byreproduction laser power increased for reproduction of informationrecorded on the first information recording layer 20 as the marks 32 andthe spaces 33 and information recorded on the second informationrecording layer 40 as the marks 52 and the spaces 53. This makes itpossible to provide a highly versatile optical information recordingmedium 1.

Further, as shown in FIG. 6, in the optical information recording medium1, the region B is located closer to the center of the opticalinformation recording medium 1 than the region A. That is, the BCA islocated in the vicinity of the center of the optical informationrecording medium 1. Thus, the disk-type identification information andthe individual identification information are recorded on a radialposition located closer to the center of the optical informationrecording medium 1 than the region A, which requires tracking forinformation reproduction.

Here, in a case where the BCA contains the disk-type identificationinformation and the individual identification number, a region having apredetermined length along the radial direction and corresponding to asingle circle along the circumferential direction is needed as a regionin which to record the BCA, so that the disk-type identificationinformation and the individual identification information, both of whichare included in the BCA, can be reproduced even if the radial directionposition of reproduction light emitted from the reproduction opticalsystem is slightly off.

Securement of such a region in which to record the disk-typeidentification information and the individual identification informationresults in a decrease in area of the information recording region(region A) in which to store other information.

However, since the radial position of the region B on which thedisk-type identification information and the individual identificationinformation are recorded is located closer to the center of the opticalinformation recording medium 1 than the region A, the decline inrecording capacity of the region A can be curbed as compared with a casewhere the radial position of the region B is located further from thecenter of the optical information recording medium 1 than the region A.

Furthermore, it is possible to decrease a volume of the BCA to berecorded on the optical information recording medium 1, as compared witha case where the BCA is located in the vicinity of the outer edge of theoptical information recording medium 1. Therefore, it is possible toreduce the amount of time required for recording the BCA on the opticalinformation recording medium 1.

(2. Problems that Occur in the Case of a Combination of ConventionalArts)

The following explains problems that occur in the case of application ofa super-resolution technology to a conventional multilayer read-onlyinformation recording medium.

Explained first is a difference in reproducing characteristic thatarises from a difference in recording format of pre-pits andconfiguration of an information recording layer of an opticalinformation recording medium based on a mark edge recording method.Explained next is the reproducing characteristic of an opticalinformation recording medium based not on the mark edge recording methodbut on a monotone recording method (equivalent to mark positionrecording method).

(2-1. Multilayer Super-Resolution Optical Information Recording MediumBased on the Mark Edge Recording Method)

(Experiment for Evaluating Reproduction Signals from a Single-LayerInformation Recording Layer and a Multilayer Information RecordingLayer)

First, the following shows, with reference to FIGS. 8 through 10, thatthe reproducing characteristic of each information recording layer of amultilayer super-resolution optical information recording medium can beenvisaged on the basis of the result of an optical information recordingmedium having a single information recording layer produced in a similarmanner to each information recording layer of the multilayersuper-resolution optical information recording medium, as long as thethickness etc. of an intermediate layer that separates the informationrecording layers from each other are appropriate.

Two types of sample, namely an optical information recording media(multilayer super-resolution optical information recording medium) 100and an optical information recording medium 200 each including aread-only information recording layer(s), were prepared, and anexperiment for evaluating reproduction signals from the respectiveinformation recording layers as obtained by reproducing informationrecorded on the two types of sample was conducted in the followingmanner.

The optical information recording medium 100 and the optical informationrecording medium 200 are different from each other in that whereas theoptical information recording medium 100 includes two informationrecording layers, the optical information recording medium 200 includesa single information recording layer. The optical information recordingmedium 100 and the optical information recording medium 200 are similarto each other except for this point.

FIG. 8 is a schematic view showing a configuration of the opticalinformation recording medium 100 of the present embodiment.

As shown in FIG. 8, the optical information recording medium 100 thusprepared had a configuration in which a cover layer 10, a firstinformation recording layer 120, an intermediate layer 30, a secondinformation recording layer 140, and a substrate 50, were stacked inthis order from an incident side on which reproduction light isincident.

It should be noted that a reproduction optical system of a reproducingdevice for reproducing information recorded on the optical informationrecording medium 100 has a reproduction wavelength λ of 405 nm and hasan N.A. of 0.85.

The cover layer 10 was made of an ultraviolet curing resin having athickness of 75 μm (with a refractive index of 1.50 at a reproductionlight wavelength of 405 nm).

The first information recording layer 120 includes (i) asuper-resolution film 123 constituted by two thin films and (ii) a groupof pre-pits 31 formed in the intermediate layer 30 put underneath thefirst information recording layer 120.

As the two thin films constituting the super-resolution film 123, areproduction film 21 and a reflection film 22 were stacked in this orderfrom an incident side on which reproduction light is incident. Thereproduction film 21 is made of zinc oxide having a thickness of 55 nm,and the reflection film 22 is made of Ti having a thickness of 6 nm. Itshould be noted that the first information recording layer 120corresponds to the aforementioned first information recording layer 20.

The intermediate layer 30 was made of a transparent ultraviolet curingresin (with a refractive index of 1.50 at the reproduction lightwavelength). The intermediate layer 30 had a thickness of 25 μm so thatan interlayer crosstalk did not occur between the first informationrecording layer 120 and the second information recording layer 140.Provided on that surface of the intermediate layer 30 which faces thefirst information recording layer 120 is the group of pre-pits 31 formedin an in-pit recording format and placed at a track pitch of 0.32 μm(see FIG. 2). That is, the first information recording layer 120 has thegroup of pre-pits 31 that are provided so that the marks are depressedwith respect to the light incident surface, and a push-pull forreproducing the information recorded on the first information recordinglayer 120 is negative in polarity.

The group of pre-pits 31 was formed in the intermediate layer 30 bycarrying out 2P transfer twice with use of, a master, a substrate that(i) was compression-molded by the same stamper as the substrate 50(described later) and (ii) was therefore substantially identical to thesubstrate 50. The phrase “carrying out 2P transfer twice” means that thefirst 2P transfer was carried out from the original master, and then thesecond 2P transfer was carried out with use of the resulting 2Ptransferred substrate as a master. Thus, the group of pre-pits 31 wasformed as depressions in accordance with information recorded in theform of shapes on the first information recording layer 120.

The second information recording layer 140 includes (i) asuper-resolution film 143 constituted by two thin films and (ii) a groupof pre-pits 51 formed in the substrate 50 put underneath the secondinformation recording layer 140.

As the two thin films constituting the super-resolution film 123, areproduction film 41 and a reflection film 42 are stacked in this orderfrom an incident side on which reproduction light is incident. Thereproduction film 41 is made of zinc oxide having a thickness of 65 nm,and the reflection film 42 is made of Ta having a thickness of 7 nm. Itshould be noted that the second information recording layer 140corresponds to the aforementioned second information recording layer 40.

The substrate 50 is a disk-shaped polycarbonate substrate having adiameter of 120 mm and a thickness of 1.1 mm. Provided on that surfaceof the substrate 50 which faces the second information recording layer140 is the group of pre-pits 51 formed in an in-pit recording format andplaced at a track pitch of 0.32 μm (see FIG. 3).

The group of pre-pits 51 are constituted by depressions and protrusionsformed in accordance with information recorded in the form of shapes onthe second information recording layer 140. Information is recorded onthe second information recording layer 140 by providing the group ofpre-pits 51 so that the marks are depressed with respect to the lightincident surface. A push-pull for reproducing the information recordedby the group of pre-pits 51 is negative in polarity.

The information recorded by the groups of pre-pits 31 and 51 isreproduced by modulating it in accordance with the 1-7RLL modulationmethod. The groups of pre-pits 31 and 51 are formed so that the marks 32and 52 have a plurality of lengths. The marks 32 and 52 are provided sothat the average length of the smallest mark and the smallest space is93 nm, which is less than or equal to the resolution limit (λ/(4N.A.)=119 nm) of the reproduction optical system.

FIG. 9 is a schematic view showing a configuration of the opticalinformation recording medium 200, which serves as a comparative example.

The optical information recording medium 200 is a single-layer opticalinformation recording medium for comparison as obtained by adapting thefirst information recording layer 120 stacked on the substrate 50 ofFIG. 8 to a single-layer disk.

As shown in FIG. 9, the optical information recording medium 200 has astructure in which a cover layer 10, an information recording layer 220,and a substrate 50 are stacked in this order from an incident side onwhich reproduction light is incident.

The information recording layer 220 includes (i) a super-resolution film223 constituted by two thin films and (ii) a group of pre-pits 51, aswith the first information recording layer 120 of FIG. 8. That is, theinformation recording layer 220 has a structure in which a reproductionfilm 21, a reflecting film 22, and the group of pre-pits 51 stacked inthis order from the incident side on which reproduction light isincident.

The substrate 50 has the group of pre-pits 51 formed on a surfacethereof which faces the information recording layer 220, as in theoptical information recording medium 100 of FIG. 8.

The dependence on reproduction power (reproducing characteristic) of abit error rate (hereinafter referred to as bER) indicating an error rateof reproduction of (i) information recorded on the first informationrecording layer 120 of the optical information recording medium 100 and(ii) the information recorded on the information recording layer 220 ofthe optical information recording medium 200 was evaluated by usingevaluation machines.

The evaluation machines used were (i) DDU-1000 (reproduction opticalsystem; reproduction light wavelength (λ)=405 nm; numerical apertureN.A.=0.85), which is a product of PULSETEC INDUSTRIAL CO., LTD. and is ageneral BD evaluation machine, (ii) a signal detector for BD evaluationmanufactured by PULSETEC INDUSTRIAL CO., LTD., and (iii) general signalprocessing PRML (12221) as high-density signal processing.

It should be noted, however, that in order to be compatible with highdensity, the signal detector for BD evaluation manufactured by PULSETECINDUSTRIAL CO., LTD. had been altered so that only the gain increased by10 dB.

FIG. 10 shows results of evaluation of information recorded on the firstinformation recording layer 120 of the optical information recordingmedium 100 and information recorded on the information recording layer220 of the optical information recording medium 200.

In FIG. 10, the horizontal axis represents reproduction laser power(i.e., reproduction light intensity) (mW) of the evaluation machine, andthe vertical axis represents bER.

As shown in FIG. 10, as a result of comparison between (i) thereproducing characteristics of the first information recording layer 120of the optical information recording medium 100 having two informationrecording layers and (ii) the reproducing characteristics of theinformation recording layer 220 of the optical information recordingmedium 200 having a single information recording layer, almost nodifference can be found between these reproducing characteristics.

That is, the first information recording layer 120 and the informationrecording layer 220 are almost identical in terms of the amount ofchange in bER relative to a change in reproduction laser powerintensity.

Accordingly, the evaluation results of FIG. 10 show that in a case wherethe intermediate layer 30 is properly provided so that an interlayercrosstalk does not occur, there is no influence on the reproducingcharacteristic of each information recording layer of thesuper-resolution multilayer optical information recording medium, exceptfor a loss in amount of light as caused by transmission of light throughthe information recording layer.

Further, although not illustrated, as a result of comparison between (i)the reproducing characteristics of the second information recordinglayer 140 of the optical information recording medium 100 and (ii) thereproducing characteristics of the information recording layer 220 ofthe optical information recording medium 200, almost no difference canbe found between these reproducing characteristics, except for thedecrease in reproduction sensitivity as caused by a light loss caused bytransmission of light through the first information recording layer 120.

This experimental result shows that whether or not the reproducingcharacteristics of each information recording layer of asuper-resolution multilayer optical information recording medium aresuperior or inferior can be determined simply by measuring thereproducing characteristics of a single-layer optical informationrecording medium having a single information recording layer.

(Relationship Between (i) the Recording Format of Pre-Pits and theConfiguration of Information Recording Layers and (ii) ReproducingCharacteristics)

For confirmation of whether reproduction on each information recordinglayer of a read-only multilayer optical information recording media issuperior or inferior according to the recording format of pre-pits andthe density of pre-pits, the following experimental optical disks, i.e.,single-layer optical information recording media 300A, 300B, 400A, 400B,500A, 500 b, 600A, and 600B were prepared in accordance with the aboveconclusion.

(a) of FIG. 11 is a schematic view showing a configuration of theoptical information recording medium 300A. (b) of FIG. 11 is a schematicview showing a configuration of the optical information recording medium300B.

(a) of FIG. 12 is a schematic view showing a configuration of each ofthe optical information recording media 400A, 500A, and 600A. (b) ofFIG. 12 is a schematic view showing a configuration of each of theoptical information recording media 400B, 500B, and 600B.

(a) of FIG. 13 is an enlarged perspective view showing a configurationof a group of pre-pits in an in-pit format. (b) of FIG. 13 is anenlarged perspective view showing a configuration of a group of pre-pitsin an on-pit format.

The optical information recording medium 300A has a structure in which acover layer 310, an information recording layer 320A, and a substrate350A are stacked in this order from an incident side on whichreproduction light is incident (see (a) of FIG. 11).

The cover layer 310 was made of an ultraviolet curing resin having athickness of 100 μm (with a refractive index of 1.50 at a reproductionlight wavelength of 405 nm).

The information recording layer 320A includes (i) a non-super-resolutionfilm 323 constituted by a single thin film and (ii) a group of pre-pits351A formed in the substrate 350A put underneath the informationrecording layer 320A. The non-super-resolution film 323 was made of Tihaving a thickness of 5 nm.

The substrate 350A was a disk-shaped polycarbonate substrate made ofpolycarbonate having a diameter of 120 mm and a thickness of 1.1 mm.

The substrate 350A had, on a surface thereof which faces thenon-super-resolution film 323, the group of pre-pits 351A placed at atrack pitch (i.e., a length along a radial direction) of 0.32 μm (see(a) of FIG. 13). It should be noted that the group of pre-pits 351A wasconstituted by depressions and protrusions formed in accordance withinformation recorded as shapes on the information recording layer 320A.

In the group of pre-pits 351A, a plurality of marks having differentlengths along a circumferential direction were disposed so thatinformation could be reproduced in accordance with the 1-7RLL modulationmethod.

The group of pre-pits 351A was provided so that the average length ofthe smallest mark and the smallest space among marks 352A and spaces353A constituted by the group of pre-pits 351A was 149 nm (whichcorresponds to 25 GB on 120-mm-diameter disk basis). That is, theaverage length of the smallest mark and the smallest space constitutedby the group of pre-pits 351A is greater than the resolution limit (λ/(4N.A.)=119 nm) of a reproduction optical system.

Furthermore, the recording format of the group of pre-pits 351A formedin the substrate 350A was an in-pit format. That is, the group ofpre-pits 351A was formed so that the marks 352A were more depressed thanthe spaces 353A with respect to the light incident surface. In otherwords, the group of pre-pits 351A was formed so that a push-pull signalin a case where the information recorded on the information recordinglayer 320A is reproduced was negative in polarity.

The optical information recording medium 300B has a structure in which acover layer 310, an information recording layer 320B, and a substrate350B are stacked in this order from an incident side on whichreproduction light is incident (see (b) of FIG. 11).

The information recording layer 320B includes (i) a non-super-resolutionfilm 323 and (ii) a group of pre-pits 351B formed in the substrate 350Bput underneath the information recording layer 320B.

The optical information recording medium 300B is identical inconfiguration to the optical information recording medium 300A exceptfor the substrate 350B (i.e., except for the recording format of thegroup of pre-pits 351B).

The substrate 350B was a disk-shaped polycarbonate substrate having adiameter of 120 mm and a thickness of 1.1 mm, as with the substrate350A.

The substrate 350B had, on a surface thereof which faces thenon-super-resolution film 323, the group of pre-pits 351B placed at atrack pitch of 0.32 μm (see (b) of FIG. 13). It should be noted that thegroup of pre-pits 351B was constituted by depressions and protrusionsformed in accordance with information recorded as shapes on theinformation recording layer 320A. The recording format of the group ofpre-pits 351B was an on-pit format.

That is, the group of pre-pits 351B was formed so that marks 352B weremore protruding than spaces 353B with respect to the light incidentsurface. In other words, the group of pre-pits 351B was formed so that apush-pull in a case where the information recorded on the informationrecording layer 320B is reproduced was positive in polarity.

The group of pre-pits 351B was formed by carrying out a 2P transfer withthe use of, as a master, a substrate that (i) was compression-molded bythe same stamper as the substrate 350A and (ii) was thereforesubstantially identical to the substrate 350A. Thus, the group ofpre-pits 351B in the on-pit format was formed in the substrate 350B.

That is, the substrates 350A and 350B were in a so-callednegative-positive relationship in photographic terms.

The optical information recording medium 400A has a structure in which acover layer 410, an information recording layer 420A, and a substrate450A are stacked in this order from an incident side on whichreproduction light was incident (see (a) of FIG. 12).

The cover layer 410 was made of an ultraviolet curing resin having athickness of 100 μm (with a refractive index of 1.50 at a reproductionlight wavelength of 405 nm).

The information recording layer 420A includes (i) a super-resolutionfilm 423 constituted by two thin films, namely a reproduction film 421and a reflecting film 422, stacked in this order from the incident sideon which reproduction light was incident and (ii) a group of pre-pits451 formed in the substrate 450A put underneath the informationrecording layer 420A.

The reproduction film 421 was made of zinc oxide having a thickness of60 nm. The reflecting film 422 was made of Ta having a thickness of 7nm.

The substrate 450A was a disk-shaped polycarbonate substrate having adiameter of 120 mm and a thickness of 1.1 mm.

The substrate 450A had, on a surface thereof which faces thesuper-resolution film 423, the group of pre-pits 451A placed at a trackpitch of 0.32 μm (see (a) of FIG. 13). It should be noted that the groupof pre-pits 451A was constituted by depressions and protrusions formedin accordance with information recorded as shapes on the informationrecording layer 420A.

In the group of pre-pits 451A, a plurality of marks having differentlengths along a circumferential direction were disposed so thatinformation could be reproduced in accordance with the 1-7RLL modulationmethod.

The group of pre-pits 451A was provided so that the average length ofthe smallest mark and the smallest space among marks 452A and spaces453A constituted by the group of pre-pits 451A was 149 nm (whichcorresponds to 25 GB on 120-mm-diameter disk basis). That is, theaverage length of the smallest mark and the smallest space constitutedby the group of pre-pits 451A is greater than the resolution limit (λ/(4N.A.)=119 nm) of a reproduction optical system.

Furthermore, the recording format of the group of pre-pits 451A formedin the substrate 450A was an in-pit format. That is, the group ofpre-pits 451A was formed so that the marks 452A were depressed withrespect to the light incident surface. In other words, the group ofpre-pits 451A was formed so that a push-pull signal in a case where theinformation recorded on the information recording layer 420A isreproduced was negative in polarity.

It should be noted that the substrate 450A is a substrate formed throughthe same master through which the substrate 350A was formed, and assuch, is substantially identical to the substrate 350A.

The optical information recording medium 400B has a structure in which acover layer 410, an information recording layer 420B, and a substrate450B are stacked in this order from an incident side on whichreproduction light was incident (see (b) of FIG. 12). The opticalinformation recording medium 400B is identical in configuration to theoptical information recording medium 400A except for the substrate 450B(i.e., except for the recording format of a group of pre-pits 451B).

The information recording layer 420B includes (i) a super-resolutionfilm 423 constituted by two thin films, namely a reproduction film 421and a reflecting film 422, stacked in this order from the incident sideon which reproduction light was incident and (ii) a group of pre-pits451B formed in the substrate 450A put underneath the informationrecording layer 420B.

The substrate 450B was a disk-shaped polycarbonate substrate having adiameter of 120 mm and a thickness of 1.1 mm as with the substrate 450A.

The substrate 450B had, on a surface thereof which faces thesuper-resolution film 423, the group of pre-pits 451B placed at a trackpitch of 0.32 μm (see (b) of FIG. 13). It should be noted that the groupof pre-pits 451B was constituted by depressions and protrusions formedin accordance with information recorded as shapes on the informationrecording layer 420. The recording format of the group of pre-pits 451Bwas an on-pit format.

That is, the group of pre-pits 451B was formed so that marks 452B weremore protruding than spaces 453B with respect to the light incidentsurface. In other words, the group of pre-pits 451B was formed so that apush-pull in a case where the information recorded on the informationrecording layer 420B is reproduced was positive in polarity.

The group of pre-pits 451B was formed by carrying out a 2P transfer withthe use of, as a master, a substrate that (i) was compression-molded bythe same stamper as the substrate 450A and (ii) was thereforesubstantially identical to the substrate 450A. Thus, the group ofpre-pits 451B in the on-pit format was formed in the substrate 450B.

That is, the substrates 450A and 450B were in a so-callednegative-positive relationship in photographic terms.

The optical information recording medium 500A has a structure in which acover layer 510, an information recording layer 520A, and a substrate550A are stacked in this order from an incident side on whichreproduction light was incident (see (a) of FIG. 12).

The cover layer 510 was made of an ultraviolet curing resin having athickness of 100 μm (with a refractive index of 1.50 at a reproductionlight wavelength of 405 nm).

The information recording layer 520A includes (i) a super-resolutionfilm 523 constituted by two thin films, namely a reproduction film 521and a reflecting film 522, stacked in this order from the incident sideon which reproduction light was incident and (ii) a group of pre-pits551A formed in the substrate 550A put underneath the informationrecording layer 520A. The reproduction film 521 was made of zinc oxidehaving a thickness of 60 nm. The reflecting film 522 was made of Tahaving a thickness of 7 nm.

The substrate 550A was a disk-shaped polycarbonate substrate having adiameter of 120 mm and a thickness of 1.1 mm.

The substrate 550A had, on a surface thereof which faces thesuper-resolution film 523, the group of pre-pits 551A placed at a trackpitch of 0.32 μm (see (a) of FIG. 13). It should be noted that the groupof pre-pits 551A was constituted by depressions and protrusions formedin accordance with information recorded as shapes on the informationrecording layer 520A.

In the group of pre-pits 551A, a plurality of marks 552A havingdifferent lengths along a circumferential direction were disposed sothat information could be reproduced in accordance with the 1-7RLLmodulation method.

The group of pre-pits 551A was provided so that the average length ofthe smallest mark and the smallest space among the marks 552A and spaces553A constituted by the group of pre-pits 551A was 113 nm (whichcorresponds to 33 GB on 120-mm-diameter disk basis). That is, theaverage length of the smallest mark and the smallest space constitutedby the group of pre-pits 551A is less than or equal to the resolutionlimit (λ/(4 N.A.)=119 nm) of a reproduction optical system.

Furthermore, the recording format of the group of pre-pits 551A formedin the substrate 550A was an in-pit format. That is, the group ofpre-pits 551A was formed so that the marks 552A were depressed withrespect to the light incident surface. In other words, the group ofpre-pits 551A was formed so that a push-pull signal in a case where theinformation recorded on the information recording layer 520A isreproduced was negative in polarity.

The optical information recording medium 500B has a structure in which acover layer 510, an information recording layer 520B, and a substrate550B are stacked in this order from an incident side on whichreproduction light was incident (see (b) of FIG. 12). The opticalinformation recording medium 500B is identical in configuration to theoptical information recording medium 500A except for the substrate 550B(i.e., except for the recording format of a group of pre-pits 551B).

The information recording layer 520B includes (i) a super-resolutionfilm 523 constituted by two thin films, namely a reproduction film 521and a reflecting film 522, stacked in this order from the incident sideon which reproduction light was incident and (ii) a group of pre-pits551B formed in the substrate 550B put underneath the informationrecording layer 520B.

The substrate 550B was a disk-shaped polycarbonate substrate having adiameter of 120 mm and a thickness of 1.1 mm, as with the substrate550A.

The substrate 550B had, on a surface thereof which faces thesuper-resolution film 523, the group of pre-pits 551B placed at a trackpitch of 0.32 μm (see (b) of FIG. 13). It should be noted that the groupof pre-pits 551B was constituted by depressions and protrusions formedin accordance with information recorded as shapes on the informationrecording layer 540B. The recording format of the group of pre-pits 551Bwas an on-pit format.

That is, the group of pre-pits 551B was formed so that marks 552B weremore protruding than spaces 553B with respect to the light incidentsurface. In other words, the group of pre-pits 551B was formed so that apush-pull in a case where the information recorded on the informationrecording layer 520B is reproduced was positive in polarity.

The group of pre-pits 551B was formed by carrying out a 2P transfer withthe use of, as a master, a substrate that (i) was compression-molded bythe same stamper as the substrate 550A and (ii) was thereforesubstantially identical to the substrate 550A. Thus, the group ofpre-pits 551B in the on-pit format was formed in the substrate 550B.

That is, the substrates 550A and 550B were in a so-callednegative-positive relationship in photographic terms.

The optical information recording medium 600A has a structure in which acover layer 610, an information recording layer 620A, and a substrate650A are stacked in this order from an incident side on whichreproduction light was incident (see (a) of FIG. 12).

The cover layer 610 was made of an ultraviolet curing resin having athickness of 100 μm (with a refractive index of 1.50 at a reproductionlight wavelength of 405 nm).

The information recording layer 620A includes (i) a super-resolutionfilm 623 constituted by two thin films, namely a reproduction film 621and a reflecting film 622, stacked in this order from the incident sideon which reproduction light was incident and (ii) a group of pre-pits651A formed in the substrate 650A put underneath the informationrecording layer 620A. The reproduction film 621 was made of zinc oxidehaving a thickness of 60 nm. The reflecting film 622 was made of Tahaving a thickness of 7 nm.

The substrate 650A was a disk-shaped polycarbonate substrate having adiameter of 120 mm and a thickness of 1.1 mm.

The substrate 650A had, on a surface thereof which faces thesuper-resolution film 623, the group of pre-pits 651A placed at a trackpitch of 0.32 μm (see (a) of FIG. 13). It should be noted that the groupof pre-pits 651A was constituted by depressions and protrusions formedin accordance with information recorded as shapes on the informationrecording layer 640.

In the group of pre-pits 651A, a plurality of marks having differentlengths along a circumferential direction were disposed so thatinformation could be reproduced in accordance with the 1-7RLL modulationmethod.

The group of pre-pits 651A was provided so that the average length ofthe smallest mark and the smallest space among marks 652A and spaces653A constituted by the group of pre-pits 651A was 83 nm (whichcorresponds to 45 GB on 120-mm-diameter disk basis). That is, theaverage length of the smallest mark and the smallest space constitutedby the group of pre-pits 651A is less than or equal to the resolutionlimit (λ/(4 N.A.)=119 nm) of a reproduction optical system.

Furthermore, the recording format of the group of pre-pits 651A formedin the substrate 650A was an in-pit format. That is, the group ofpre-pits 651A was formed so that the marks 652A were depressed withrespect to the light incident surface. In other words, the group ofpre-pits 651A was formed so that a push-pull signal in a case where theinformation recorded on the information recording layer 620A isreproduced is negative in polarity.

The optical information recording medium 600B has a structure in which acover layer 610, an information recording layer 620B, and a substrate650B are stacked in this order from an incident side on whichreproduction light was incident (see (b) of FIG. 12). The opticalinformation recording medium 600B is identical in configuration to theoptical information recording medium 600A except for the substrate 650B(i.e., except for the recording format of a group of pre-pits 651B).

The information recording layer 620B includes (i) a super-resolutionfilm 623 constituted by two thin films, namely a reproduction film 621and a reflecting film 622, stacked in this order from the incident sideon which reproduction light was incident and (ii) a group of pre-pits651B formed in the substrate 650B put underneath the informationrecording layer 620B.

The substrate 650B was a disk-shaped polycarbonate substrate having adiameter of 120 mm and a thickness of 1.1 mm, as with the substrate650A.

The substrate 650B had, on a surface thereof which faces thesuper-resolution film 623, the group of pre-pits 651B placed at a trackpitch of 0.32 μm (see (b) of FIG. 13). It should be noted that the groupof pre-pits 651B was constituted by depressions and protrusions formedin accordance with information recorded as shapes on the informationrecording layer 620B. The recording format of the group of pre-pits 651was an on-pit format.

That is, the group of pre-pits 651B was formed so that marks 652B wereformed more protruding than spaces 653B with respect to the lightincident surface. In other words, the group of pre-pits 651B was formedso that a push-pull in a case where the information recorded on theinformation recording layer 620B is reproduced is positive in polarity.

The group of pre-pits 651B was formed by carrying out a 2P transfer withthe use of, as a master, a substrate that (i) was compression-molded bythe same stamper as the substrate 650A and (ii) was thereforesubstantially identical to the substrate 650A. Thus, the group ofpre-pits 651B in the on-pit format was formed in the substrate 650B.

That is, the substrates 650A and 650B were in a so-callednegative-positive relationship in photographic terms.

FIG. 14 shows results of measurement of reproduction signals ofexperimental optical information recording media.

Whether the reproducing characteristics of the experimental single-layeroptical information recording media 300A to 600B are superior orinferior was determined with reference FIG. 14 by measuring a bottomlevel of jitter (commonly-used criterion for evaluation of signalreproducing characteristics in a low density) or of bER (bit error rate)in each of the optical information recording media 300A to 600B with useof the aforementioned evaluation machines and the like.

FIG. 14 shows (i) results (jitter or bER) of measurement performed onthe experimental optical information recording media and (ii) signalwaveforms obtained in the measurement. In each oscilloscopic imageshowing a signal waveform in FIG. 14, the horizontal axis indicates timeand the vertical axis indicates voltage.

First, a comparison is made among the optical information recordingmedia 500A, 500B, 600A, and 600B each provided with a group of pre-pitshaving a mark length that is less than or equal to the resolution limit(λ/(4 N.A.)=119 nm) (which corresponds to 33 GB on 120-mm-diameter diskbasis) of a reproduction optical system.

As described above, the only difference between the optical informationrecording medium 500A and the optical information recording medium 500Bis that the depressions and protrusions of the group of pre-pits 551Aare in converse relation with the depressions and protrusions of thegroup of pre-pits 551B. Similarly, the only difference between theoptical information recording medium 600A and the optical informationrecording medium 600B is that the depressions and protrusions of thegroup of pre-pits 651A are in converse relation with the depressions andprotrusions of the group of pre-pits 651B.

However, as is clear from FIG. 14, a comparison of reproducingcharacteristics between the optical information recording medium 500Aand the optical information recording medium 500B shows that the jitterof the optical information recording medium 500B took on a bottom levelof 27.5%, whereas the jitter of the optical information recording medium500A took on a bottom level of 10.9%, i.e., that the jitter of theoptical information recording medium 500A is took on a lower bottomlevel than the jitter of the optical information recording medium 500B.

It was thus found that the optical information recording medium 500A hadsuperior reproducing characteristics to the optical informationrecording medium 500B.

Similarly, a comparison of reproducing characteristics between theoptical information recording medium 600A and the optical informationrecording medium 600B shows that the bER of the optical informationrecording medium 600A took on a bottom level of 3.6×10⁻⁵, which issufficiently lower than 3.0×10⁻⁴, which is generally required forpractical use, whereas the optical information recording medium 600Bends up being incapable of even generating clocks minimum required fordecoding a reproduction signal.

It was thus found that the optical information recording medium 600A hadsuperior reproducing characteristics to the optical informationrecording medium 600B.

Accordingly, it was found that even in the case of use of substrateshaving groups of pre-pits identical to each other in terms of thedensity of marks, reproducing characteristics varied significantlydepending on whether the recording format of the groups of pre-pits wasan on-pit format or an in-pit format. It was found that an in-pit formatgives better reproducing characteristics than an on-pit format.

Further, FIG. 14 shows that the larger the density (informationrecording density) of marks is, the greater the difference betweenreproducing characteristics obtained in an in-pit format and reproducingcharacteristics obtained in an on-pit format is.

In particular, it was found that when the average length of the smallestmark (mark 652B) and the smallest space (space 653B) is 83 nm (=λ/(5.76NA (which corresponds to 45 GB on a 120-mm-diameter disk basis))) as inthe case of the group of pre-pits 651B of the optical informationrecording medium 600B, it becomes impossible to even generate a clock.

That is, when the average length of the smallest mark and the smallestspace among the group of pre-pits 651 is 83 nm (λ/(5.76 NA)), it isimpossible to generate a clock and, accordingly, it is impossible toimprove bER (i.e., reproducing characteristic) by upgrading signalprocessing (e.g., changing from PRML (1221) to PRML (12221), etc.).

Next, as can be seen from FIG. 14, a comparison of reproducingcharacteristics between (i) the optical information recording media 300Aand 400A and (ii) the optical information recording media 300B and 400Bshowed that the jitter of the optical information recording medium 300Atook on an approximately equal bottom level to the jitter of the opticalinformation recording medium 300B and that the jitter of the opticalinformation recording medium 400A took on an approximately equal bottomlevel to the jitter of the optical information recording medium 400B. Itwas thus found that there was no big difference in reproducingcharacteristic between the optical information recording media 300A and400A and the optical information recording media 300B and 400B.

That is, among the optical information recording media 300A, 300B, 400A,and 400B (e.g., which corresponds to 25 GB on a 120-mm-diameter diskbasis) having the respective groups of pre-pits (the group of pre-pits351A, the group of pre-pits 351B, the group of pre-pits 451A, and thegroup of pre-pits 451B, respectively), among each of which the averagelength between the smallest mark and the smallest space was greater thanthe resolution limit (λ/(4 N.A.)=119 nm) of a reproduction opticalsystem, there was no big difference in reproducing characteristicbetween an on-pit format and an in-pit format, regardless of whether theinformation recording layers were super-resolution films (theinformation recording layers 420A, 420B, 520A, 520B, 620A, and 620B) ornon-super-resolution films (the information recording layers 320A and320B).

This showed that the reason for better reproducing characteristics thatare obtained in a case where the recording format of a group of pre-pitsis an in-pit format is that reproducing characteristics depend on therecording density (i.e., the average length of the smallest mark and thesmallest space among the group of pre-pits), regardless of the type ofinformation recording layer (whether the information recording layer isa super-resolution film or a non-super-resolution film).

This showed that in order to reproduce information recorded on anoptical information recording medium provided with a group of pre-pitshaving a mark length less than or equal to the resolution limit (λ/(4N.A.)=119 nm) of a reproduction optical system (which corresponds to 33GB on a 120-mm-diameter disk basis), use of an in-pit format as therecording format of the group of pre-pits, regardless of whether theinformation recording layer is a super-resolution film or anon-super-resolution film, allowed achieving better reproducingcharacteristics as compared with a case in which the recording format ofthe group of pre-pits is an on-pit format.

(2-2. Comparison with a Multilayer Super-Resolution Optical InformationRecording Medium Based on a Monotone Recording Method)

Next, results of comparison of reproducing characteristic between theoptical information recording medium according to the present embodimentand a multilayer super-resolution optical information recording mediumbased on the monotone recording method are explained with reference toFIGS. 15 through 17.

FIG. 15 is a schematic view showing a configuration of an opticalinformation recording medium 700 having pre-pits formed in a monotoneformat.

(a) of FIG. 16 is an enlarged perspective view showing a configurationof a group of pre-pits in a monotone on-pit format, and (b) of FIG. 16is an enlarged perspective view showing a configuration of a group ofpre-pits in a monotone in-pit format.

As shown in FIG. 15, the optical information recording medium 700 has astructure in which a cover layer 710, a first information recordinglayer 720, an intermediate layer 730, a second information recordinglayer 740, and a substrate 750 are stacked in this order from anincident surface side on which reproduction light is incident.

The cover layer 710 is constituted by two layers: a polycarbonate film711 having a film thickness of 80 μm and a transparent adhesive resinlayer having a film thickness of 20 μm. The polycarbonate film 711 andthe transparent adhesive resin layer are stacked in this order from theincident side on which reproduction light is incident.

The first information recording layer 720 is constituted by (i) asuper-resolution film 723 constituted by two thin films, namely areproduction film 721 and a reflecting film 722, which are stacked inthis order from the incident side on which reproduction light isincident and (ii) a group of pre-pits 731 formed in an intermediatelayer 730 put underneath the first information recording layer 720.

The reproduction film 721 was made of zinc oxide having a thickness of175 nm, and the semi-transparent film 722 was made of Si having athickness of 7 nm.

The intermediate layer 730 was made of a transparent ultraviolet curingresin having a thickness of 25 μm (with a refractive index of 1.50 atthe reproduction light wavelength).

As shown in (a) of FIG. 16, the intermediate layer 730 had, on a surfacethereof which faces the super-resolution film 723, the group of pre-pits731, placed at a track pitch of 0.32 μm, whose recoding format was anon-pit recording format in accordance with information recorded asshapes on the first information recording layer 720.

The group of pre-pits 731 were formed by carrying of 2P transfer withthe use of, as a master, a substrate that (i) was compression-molded bythe same stamper as a substrate 750 (described later) and (ii) wastherefore substantially identical to the substrate 750. Thus, the groupof pre-pits 731 in the monotone on-pit format was formed in theintermediate layer 730. The group of pre-pits 731 are divided into eightregions.

In the example shown in (a) of FIG. 16, the group of pre-pits 731 isdivided into eight regions along a radial direction. The average lengthof the length (mark length) of a mark 732 and the length (space length)of a space 733 in each of the regions is 60 nm, 80 nm, 100 nm, 120 nm,140 nm, 160 nm, 200 nm, and 400 nm in the order of the eight regionsalong the radial direction.

As such, the group of pre-pits 731 is a group of pre-pits in a monotoneformat with eight different lengths (equivalent to densities), thusmaking it possible to determine reproducing characteristics in each ofthe regions.

As shown in FIG. 15, the second information recording layer 740 isconstituted by (i) a super-resolution film 743 constituted by two thinfilms and (ii) a group of pre-pits 751 formed in the substrate 750 putunderneath the second information recording layer 740. Thesuper-resolution film 743 is constituted by (i) a reproduction film 741,which is made of zinc oxide having a thickness of 155 nm and (ii) areflecting film 742, which is made of Si having a thickness of 50 nm.The reproduction film 741 and the reflecting film 742 are stacked inthis order from the incident side on which reproduction light isincident.

The substrate 750 used was a disk-shaped polyolefin substrate having thegroup of pre-pits 751.

As shown in (b) of FIG. 16, the substrate 750 has, on a surface thereoffaces the super-resolution film 743, the group of pre-pits 751, placedat a track pitch of 0.32 μm, whose recording format was an in-pitrecording format in accordance with information recorded as shapes onthe second information recording layer 740. The group of pre-pits 751are divided into eight regions.

In the example shown in (b) of FIG. 16, the group of pre-pits 751 isdivided into eight regions along the radial direction. The averagelength of the length (mark length) of a mark 752 and the length (spacelength) of a space 753 in each of the regions is 60 nm, 80 nm, 100 nm,120 nm, 140 nm, 160 nm, 200 nm, and 400 nm in the order of the eightregions along radial direction.

As such, the group of pre-pits 751 is a group of pre-pits in a monotoneformat with eight different lengths (equivalent to densities), thusmaking it possible to determine reproducing characteristics in each ofthe regions.

Information recorded on the first information recording layer 720 of theoptical information recording medium 700 and information recorded on thesecond information recording layer 740 of the optical informationrecording medium 700 were reproduced by an evaluation device having areproduction optical system having a reproduction light wavelength of404 nm and a numerical aperture of 0.85, and OTFs were measured.

FIG. 17 shows the reproducing characteristics (OTF) of the first andsecond information recording layers 720 and 740 of the opticalinformation recording medium 700 having the group of monotone pre-pits.

Note that OTF is an index that indicates super-resolution performance.OTF indicates the dependence of C/N (evaluation criterion indicatingsignal quality) the recording mark length (equivalent to the pit lengthin the case of a read-only optical information recording medium).

As is clear from FIG. 17, there is no significant difference inreproducing characteristic between the first information recording layer720 and the second information recording layer 740.

That is, results shown in FIG. 17 show that in a multilayersuper-resolution optical information recording medium based on themonotone recording method, such as the optical information recordingmedium 700, reproducing characteristics do not vary depending on therecording format (i.e., in-pit or on-pit) of pre-pits.

As is clear from the foregoing description, a common multilayer opticalinformation recording medium employs a mark edge recording method forimprovement in storage capacity. Furthermore, for reduction of cost, (i)a group of pre-pits provided for recording information on a firstinformation recording layer located closer to a reproduction lightincident surface than a second information layer is and (ii) a group ofpre-pits provided for recording information on the second informationrecording layer, which is located further from the reproduction lightincident surface than the first information recording layer, are indifferent recording formats: an in-pit format and an on-pit format.

For this reason, application of the super-resolution technology to themultilayer optical information recording medium causes one of theinformation recording layers in which information is recorded in anon-pit format to have significantly inferior reproducing characteristicsto the other of the information recording layers in which information isrecorded in an in-pit format.

On the other hand, as described above, in the optical informationrecording medium 1 according to the present embodiment, the recordingformat of the group of pre-pits 51, among which the average length ofthe smallest mark and the smallest space is less than the resolutionlimit (λ/(4 N.A.)=119 nm) of a reproduction optical system, is an in-pitformat. In addition, the second information recording layer 40, which isa super-resolution film that makes it possible to reproduce informationrecorded by the group of pre-pits 51, is provided on the substrate 50provided with the group of pre-pits 51.

For this reason, there is no difference in characteristic between asignal reproduced on the second information recording layer 40 and asignal reproduced on the first information recording layer 20. Thisenables excellent, high-density signal reproduction with highreliability. Further, it becomes possible to provide an inexpensive andhigh-capacity multilayer optical information recording medium based on asuper-resolution technology.

In a case where the average length of the smallest mark 52 and thesmallest space 53 among the group of pre-pits 51 is less than or equalto λ/(5.76 NA), the optical information recording medium 1 of thepresent embodiment is especially preferable.

This is because, as described above, a difference in reproducingcharacteristic between the in-pit format and the on-pit format becomesmore significant as the average length of the smallest mark and thesmallest space among the group of pre-pits 31 (51) becomes shorter thanthe resolution limit length of the reproduction optical system. Inparticular, in a case where the average length of the smallest mark andthe smallest space is less than or equal to λ/(5.76 NA), the generationof clocks, which is essential for carrying out signal processing forcomplementing the reproducing characteristics, becomes impossible.

Note that if clocks can be generated, signal processing methods (e.g.,combined use of PRML (1221) and PRML (12221), which has a highercomplementarity than that of PRML (1221), etc.) having different degreesof complementarity can be used depending on a signal reproduced.However, in a case where such a method is used, there is still such aproblem that the optical information recording medium reproducing devicebecomes so complex in structure that there is an increase in cost.

On the other hand, the configuration of the optical informationrecording medium 1 makes it possible to prevent such a problem thatclocks cannot be generated. For this reason, a reproduced signal can becomplemented by a single signal processing method. This makes itpossible to prevent the optical information recording medium reproducingdevice from being complex in structure, thus bringing about an effect ofsuppressing an increase in cost.

It should be noted that the optical information recording medium 1 isnot limited to the above-described configuration in which both the groupof pre-pits 31 and the group of pre-pits 51 are in an in-pit format. Theoptical information recording medium 1 can employ a configuration inwhich either the group of pre-pits 31 or the group of pre-pits 51 is inan in-pit format.

Furthermore, either of the first and second information recording layers20 and 40 may be a layer (RE layer) on which information can berewritten, instead of being a read-only ROM layer.

Furthermore, an optical information recording medium according to anembodiment of the present invention is not limited to any one theoptical information recording media described above, and can be aninformation recording medium constituted by three or more layers byfurther including various types of information recording layer. In thiscase, the cover layer and the intermediate layer may be different inthickness from the those described above.

The present invention is not limited to the description of theembodiments above, but may be altered by a skilled person within thescope of the claims. An embodiment based on a proper combination oftechnical means disclosed in different embodiments is encompassed in thetechnical scope of the present invention.

As described above, an optical information recording medium of thepresent invention includes: a light transmittable layer having anincident surface on which reproduction light is incident; two or moreinformation recording layers; a substrate, the light transmittablelayer, the information recording layers, and the substrate being stackedin this order from an incident side on which the reproduction light isincident; and an intermediate layer that separates the informationrecording layers from each other, the two or more information recordinglayers having information recorded thereon as marks and spaces by apredetermined modulation method, the two or more information recordinglayers each including (i) a group of pre-pits constituting the marks andthe spaces and (ii) a super-resolution film, the marks and the spacesconstituted by the group of pre-pits having different lengths, anaverage length of a smallest mark that is smallest in length among themarks constituted by the group of pre-pits and a smallest space that issmallest in length among the space constituted by the group of pre-pitsis less than or equal to a resolution limit of a reproduction opticalsystem for reproducing the information recorded on the informationrecording layers, the resolution film being a film that enables thereproduction optical system to reproduce information recorded by thegroup of pre-pits, the group of pre-pits being formed so that apush-pull signal for the reproduction optical system to reproduce theinformation recorded by the group of pre-pits is negative in polarity.

The group of pre-pits here is constituted by a plurality of pre-pits.The pre-pits mean depressed and protruding shapes provided in and on thesubstrate and the intermediate layer.

Further, it is known that the resolution limit of the optical system forreading (reproducing) the information recoded as the marks and thespaces is approximately λ/(4 NA), where λ is the reproduction lightwavelength of the reproduction optical system and NA is the numericalaperture of the reproduction optical system. For this reason, thepresent application assumes that the resolution limit of thereproduction optical system for reproducing the information recorded onthe information recording layers is λ/(4 NA).

Further, the average length of the smallest mark and the smallest spaceis a length that can be calculated from the predetermined modulationmethod and the density of the information recorded on the informationrecording layers. Structurally, for example, in the case of the 1-7RLLmodulation method, the average length is an average length of thesmallest mark, 2T mark length, and the smallest space, 2T space length.

According to the foregoing configuration, the two or more informationrecording layers separated from each other by the intermediate layerhave information recorded thereon as marks and spaces by a predeterminedmodulation method. This allows the information thus recorded to bereproduced by a reproducing device capable of modulation by apredetermined modulation method.

According to the foregoing configuration, the two or more informationrecording layers each include a group of pre-pits constituting the marksand the spaces. That is, the information recording layer formed on theintermediate layer or on the substrate to include a group of pre-pitshas information recorded thereon in the form of depressions andprotrusions of the group of pre-pits. Such an information recordinglayer including a group of pre-pits is a read-only information recordinglayer.

According to the foregoing configuration, the marks constituted by thegroup of pre-pits have different lengths. By thus causing the marks tohave different lengths, an improvement in density at which informationis recorded can be achieved as compared with a so-called monotonepattern recording method (i.e., a mark position recording method) wheremarks have the same length.

According to the foregoing configuration, an average length of asmallest mark that is smallest in length among the marks constituted bythe group of pre-pits and a smallest space that is smallest in lengthamong the space constituted by the group of pre-pits is less than orequal to a resolution limit of a reproduction optical system forreproducing the information recorded on each of the informationrecording layers. This makes it possible to improve the density at whichthe marks and the spaces are disposed, thus making it possible to recorda large volume of information on each of the information recordinglayers.

This makes it possible to suppress an increase in the number ofinformation recording layers to be formed for an increase in volume ofinformation to be recorded, thus making it possible to suppress anincrease in manufacturing cost due to an increase in the number ofinformation recording layers to be stacked.

According to the foregoing configuration, the resolution film is a filmthat enables the reproduction optical system to reproduce informationrecorded by the group of pre-pits. This allows the information recordedon the information recording layers to be reproduced by the reproductionoptical system serving as a reproducing device.

It should be noted here that of optical information recording media,such an optical information recording media having two or moreinformation recording layers stacked, with each information recordinglayer including (i) a group of pre-pits in which the average length ofthe smallest mark and the smallest space is less than or equal to theresolution limit of a reproduction optical system and (ii) asuper-resolution film that is a film that enables the reproductionoptical system to reproduce information recorded by the group ofpre-pits is sometimes referred to as “multilayer super-resolutionoptical information recording medium”.

A multilayer super-resolution optical information recording medium thusconfigured allows the reproduction optical system to reproduce a largevolume of information recorded on each information recording layer ofthe multilayer super-resolution optical information recording medium.

By thus configuring a multilayer super-resolution optical informationrecording medium, an increase in the number of information recordinglayers to be formed for an increase in volume of information to berecorded. This makes it possible to suppress an increase inmanufacturing cost due to an increase in the number of informationrecording layers to be stacked.

According to the foregoing configuration, the group of pre-pits isdisposed so that a push-pull signal for the reproduction optical systemto reproduce the information recorded by the group of pre-pits isnegative in polarity. Such a group of pre-pits that a push-pull signalfor the reproduction optical system to reproduce the informationrecorded by the group of pre-pits is negative in polarity has eachpre-pit formed in an in-pit format.

It should be noted that the in-pit format is a format by which the marksare formed more depressed than the spaces with respect to the incidentsurface on which the reproduction light is incident.

This makes it possible to, even if the marks and the spaces are disposedso that the average length of the smallest mark and the smallest spaceis less than or equal to the resolution limit of the reproductionoptical system, prevent deterioration of the reproducing characteristicsof information that is obtained by the reproduction optical systemreproducing the information recorded on the information recordinglayers.

Thus, the foregoing configuration makes it possible to provide aninexpensive and high-capacity optical information recording medium thatis prevented from deteriorating in reproducing characteristic.

The optical information recording medium of the present invention ispreferably configured such that the average length of the smallest markand the smallest space among the group of pre-pits is less than or equalto λ/(5.76 NA), where λ is the reproduction light wavelength of thereproduction optical system and NA is the numerical aperture of thereproduction optical system.

In a case where the average length of the smallest mark and the smallestspace in the group of pre-pits is greater than λ/(5.76 NA), thegeneration of clocks for reproducing information recorded on theinformation recording layer is possible even if the recording format ofthe pre-pits is an on-pit format, as will be mentioned later.

This makes it possible to obtain necessary reproducing characteristicsby complementing reproducing characteristics even if the reproducingcharacteristics deteriorate.

That is, when the average length of the smallest mark and the smallestspace in the group of pre-pits is less than or equal to λ/(5.76 NA),there is a remarkable difference in reproducing characteristic dependingon the recording format (in-pit recording format or on-pit recordingformat) of the group of pre-pits.

According to the foregoing configuration, even if the average length ofthe smallest mark and the smallest space in the group of pre-pits isless than or equal to λ/(5.76 NA), the information recorded on theinformation recording layer by the group of pre-pits can be reproduced.This makes it possible to prevent deterioration of reproducingcharacteristics, obtained satisfactory reproducing characteristics, andimprove recording capacity.

The optical information recording medium of the present invention ispreferably configured such that: each of the two or more informationrecording layers is indicated by information to be a layer configured toenable the reproduction optical system to reproduce the informationrecorded by the group of pre-pits, the information being contained indisk-type identification information indicating a type of the opticalinformation recording medium; and the disk-type identificationinformation is recorded on either of the two or more informationrecording layers by a recording method that renders the disk-typeidentification information more easily detectable than the informationrecorded as the marks and the spaces.

According to the foregoing configuration, each of the two or moreinformation recording layers is indicated by information to be a layerconfigured to enable the reproduction optical system to reproduce theinformation recorded by the group of pre-pits, the information beingcontained in disk-type identification information indicating a type ofthe optical information recording medium.

That is, the disk-type identification information contains informationindicating that the optical information recording medium is a multilayersuper-resolution optical information recording medium.

Moreover, the disk-type identification information is recorded on aninformation recording layer stacked on the group of pre-pits by arecording method that renders the disk-type identification informationmore easily detectable than the information recorded as the marks andthe spaces.

In the case of reproduction of the information recorded as the marks andthe spaces on the multilayer super-resolution optical recording medium,it is necessary to make reproduction laser power (intensity of thereproduction light) larger than in the case of reproduction ofinformation recorded on an optical information recording medium(non-multilayer super-resolution optical information recording medium)having marks and spaces formed at a higher density than the multilayersuper-resolution optical recording medium.

For this reason, an attempt to reproduce information on a non-multilayersuper-resolution optical information recording medium by reproductionlaser power by which to reproduce information on the multilayersuper-resolution optical recording medium may result in destruction ofthe non-multilayer super-resolution optical information recordingmedium.

The foregoing configuration makes it possible to determine whether ornot the optical information recording medium is a multilayersuper-resolution optical information recording medium by confirmingdisk-type identification information before increasing reproductionlaser power by which to reproduce information recorded as the marks andthe spaces on the multilayer super-resolution optical recording medium.

This prevents information on a non-multilayer super-resolution opticalinformation recording medium from being mistakenly reproduced byreproduction laser power increased for reproduction of informationrecorded on an information recording layer as the marks and the spaces.This makes it possible to provide a highly versatile optical informationrecording medium.

The optical information recording medium of the present invention ispreferably configured such that the recording method is a method forrecording information indicated by a plurality of stripes formed byirradiating the information recording layer with pulse laser light, theplurality of stripes having widths in units of 10 μm and lengths inunits of 100 μm to units of mm.

The foregoing configuration allows the reproduction optical system toread the disk-type identification information and an individualidentification number even if a radial direction position of focusing orreproduction light irradiation is slightly off while the reproductionoptical system is carrying out reproduction to read the disk-typeidentification information and the individual identification number.

Further, use of a dedicated pulse laser light irradiation device makesit possible to comparatively easily record disk-type identificationinformation and an individual identification number.

The optical information recording medium of the present invention ispreferably configured such that the disk-type identification informationand an individual identification number are recorded on a radialposition located closer to a center of the optical information recordingmedium than an information recording region that requires tracking forinformation reproduction, the individual identification number beinginformation for individually identifying the optical informationrecording medium.

Recording of the disk-type identification information and the individualidentification number by a recording method that allows easy detectionmakes it possible to reproduce the disk-type identification informationand the individual identification number even if the radial directionposition of reproduction light irradiation is slightly off. As a regionin which to record the disk-type identification information and theindividual identification number, a region having a predetermined lengthalong the radial direction and corresponding to a single circle alongthe circumferential direction.

Securement of such a region in which to record the disk-typeidentification information and the individual identification numberresults in a decrease in information recording region in which to storeother information.

According to the foregoing configuration, the radial position on whichthe disk-type identification information and the individualidentification number are recorded is located closer to the center ofthe optical information recording medium than the information recordingregion that requires tracking for information reproduction. The declinein recording capacity of the information recording region that requirestracking can be curbed as compared with a case where the radial positionis located further from the center of the optical information recordingmedium than the information recording region.

An optical information recording medium of the present inventionincludes: a light transmittable layer having an incident surface onwhich reproduction light is incident; two or more information recordinglayers; a substrate, the light transmittable layer, the informationrecording layers, and the substrate being stacked in this order from anincident side on which the reproduction light is incident; and anintermediate layer that separates the information recording layers fromeach other, the two or more information recording layers havinginformation recorded thereon as marks and spaces by a predeterminedmodulation method, the two or more information recording layers eachincluding (i) a group of pre-pits constituting the marks and the spacesand (ii) a super-resolution film, the marks and the spaces constitutedby the group of pre-pits having different lengths, an average length ofa smallest mark that is smallest in length among the marks constitutedby the group of pre-pits and a smallest space that is smallest in lengthamong the space constituted by the group of pre-pits being less than orequal to a resolution limit of a reproduction optical system forreproducing the information recorded on the information recordinglayers, the resolution film being a film that enables the reproductionoptical system to reproduce information recorded by the group ofpre-pits, the group of pre-pits being in an in-pit format by which themarks are formed more depressed than the spaces with respect to theincident surface on which the reproduction light is incident.

According to the foregoing configuration, the group of pre-pits is in anin-pit format by which the marks are formed more depressed than thespaces with respect to the incident surface on which the reproductionlight is incident. This makes it possible to, even if the marks and thespaces are disposed so that the average length of the smallest mark andthe smallest space is less than or equal to the resolution limit of thereproduction optical system, prevent deterioration of the reproducingcharacteristics of information that is obtained by the reproductionoptical system reproducing the information recorded on the informationrecording layers.

INDUSTRIAL APPLICABILITY

The present invention is applicable in particular to a read-only opticalinformation recording medium including a plurality of informationrecording layers and having a high recording density of information.

REFERENCE SIGNS LIST

-   -   1, 1 a, 100 Optical information recording medium    -   10 Cover layer (light transmittable layer)    -   20, 120 First information recording layer    -   23, 43 Super-resolution film    -   30 Intermediate layer    -   31, 51, 51 a Group of pre-pits    -   32, 52 Mark    -   33, 53 Space    -   40, 40 a, 140 Second information recording layer    -   50, 50 a Substrate

The invention claimed is:
 1. An optical information recording mediumreproducing device for reproducing an optical information recordingmedium, the optical information recording medium including: a lighttransmittable layer having an incident surface on which reproductionlight is incident; two or more information recording layers; asubstrate, the light transmittable layer, the information recordinglayers, and the substrate being stacked in this order from an incidentside on which the reproduction light is incident; and an intermediatelayer that separates the information recording layers from each other,the two or more information recording layers having information recordedthereon as marks and spaces by a predetermined modulation method, thetwo or more information recording layers each including (i) a group ofpre-pits constituting the marks and the spaces and (ii) asuper-resolution film, the marks and the spaces constituted by the groupof pre-pits having different lengths, an average length of a smallestmark that is smallest in length among the marks constituted by the groupof pre-pits and a smallest space that is smallest in length among thespace constituted by the group of pre-pits being less than or equal to aresolution limit of a reproduction optical system for reproducing theinformation recorded on the information recording layers, the resolutionfilm being a film that enables the reproduction optical system toreproduce information recorded by the group of pre-pits, the group ofpre-pits being in an in-pit format by which the marks are formed moredepressed than the spaces with respect to the incident surface on whichthe reproduction light is incident, each of the two or more informationrecording layers being indicated by information to be a layer configuredto enable the reproduction optical system to reproduce the informationrecorded by the group of pre-pits, the information being contained indisk-type identification information indicating a type of the opticalinformation recording medium, the disk-type identification informationbeing recorded on either of the two or more information recording layersby a recording method that renders the disk-type identificationinformation more easily detectable than the information recorded as themarks and the spaces, the optical information recording mediumreproducing device comprising: a disk-type identification confirmingsection which detects the disk-type identification information andconfirms a disk type of the disk-type identification information; and aclock generating section which generates a clock for reproducing theinformation recorded by the group of pre-pits.